Split fields related functionality out of `Distribution.Parsec`
namespace which is not left only to handle "scalar" values.
This separation highlights that we use `parsec` for *structure* (think
JSON), and the *values* of the fields (think e.g. dates in JSON).

- New top-level `Distribution.Fields` and `Distribution.Parsec` modules
  should include most things most people would need.
- Also `Distribution.Pretty.Field` is moved to `Distribution.Fields.Pretty`,
  as now it has proper place.
- The commit is moving things around and fixing compilation errors.
- I fixed `cabal check` parse error printing as I was nearby.

This changes the default behaviour to succeed when no test suites are found. The
old behaviour can be achieved by passing --test-fail-when-no-test-suites.

Sessions with new changes:

$ cabal v2-test
Resolving dependencies...
No tests to run for the package initsample-0.1.0.0

$ cabal v2-test --test-fail-when-no-test-suites
Resolving dependencies...
cabal: Cannot run tests for the target '' which refers to the package
initsample-0.1.0.0 because it does not contain any test suites.

Sometimes, rerunning the solver to remove one variable from the conflict set
also removes other variables.  This commit checks whether each variable is still
present in the conflict set before trying to remove it, which can save expensive
solver reruns.

Minimizing the conflict set requires rerunning the solver multiple times, which
can be time consuming.  This commit adds a flag, --minimize-conflict-set, that
defaults to false.  We should probably add a way to limit the total run time
before turning the feature on by default, such as applying the backjump limit to
the combined solver runs.

This commit improves the error message in the case where the solver finds no
solution after an exhaustive search.  Previously, cabal generated the error
message by rerunning the solver and having it prefer goals from the final
conflict set, so that the error message only mentioned conflicts between
packages from the final conflict set.  Conflicts relating to the final conflict
set are more likely to be relevant, because the conflict set contains all
variables that are involved in one conflict that makes the dependencies
unsatisfiable.  However, the conflict set can also include variables that led to
some conflicts but aren't relevant to the main conflict.  This commit improves
the error message further by first trying to reduce the size of the conflict
set.

The current algorithm for reducing the conflict set simply reruns the solver
with different goal orders, so it has some downsides:

- Reducing the conflict set can be slow.  It is also possible for the solver to
  reach the backjump limit on a rerun, even though the first run completed.  In
  that case, it uses the original conflict set.

- The function can fail to remove some unnecessary variables from the conflict
  set.  In the worst case, it returns the original conflict set.

I tested the feature on the example in
https://www.reddit.com/r/haskell/comments/9rmh9s/how_to_read_cabal_solver_failure_output/
and #5647.  Both runs used the command
"cabal install --only-dependencies --enable-tests --force-reinstalls --index-state=2018-10-26T20:30:16Z".

Before:

    Resolving dependencies...
    cabal: Could not resolve dependencies:
    [__0] trying: opaleye-0.6.7003.0 (user goal)
    [__1] rejecting: opaleye:!test (constraint from config file, command line
    flag, or user target requires opposite flag selection)
    [__1] trying: opaleye:*test
    [__2] trying: dotenv-0.6.0.3 (dependency of opaleye *test)
    [__3] trying: transformers-0.5.5.0/installed-0.5... (dependency of opaleye)
    [__4] next goal: contravariant (dependency of opaleye)
    [__4] rejecting: contravariant-1.5 (conflict: opaleye => contravariant>=1.2 &&
    <1.5)
    [__4] rejecting: contravariant-1.4.1, contravariant-1.4 (conflict:
    transformers => base==4.12.0.0/installed-4.1..., contravariant => base<4.12)
    [__4] rejecting: contravariant-1.3.3, contravariant-1.3.2,
    contravariant-1.3.1.1, contravariant-1.3.1, contravariant-1.3,
    contravariant-1.2.2.1, contravariant-1.2.2, contravariant-1.2.1,
    contravariant-1.2.0.1, contravariant-1.2 (conflict:
    transformers==0.5.5.0/installed-0.5..., contravariant => transformers>=0.2 &&
    <0.5)
    [__4] rejecting: contravariant-1.1, contravariant-1.0, contravariant-0.6.1.1,
    contravariant-0.6.1, contravariant-0.6, contravariant-0.5.2,
    contravariant-0.5.1, contravariant-0.5, contravariant-0.4.4,
    contravariant-0.4.3, contravariant-0.4.1, contravariant-0.4,
    contravariant-0.3, contravariant-0.2.0.2, contravariant-0.2.0.1,
    contravariant-0.2, contravariant-0.1.3, contravariant-0.1.2.1,
    contravariant-0.1.2, contravariant-0.1.1, contravariant-0.1.0.1,
    contravariant-0.1.0 (conflict: opaleye => contravariant>=1.2 && <1.5)
    [__4] fail (backjumping, conflict set: contravariant, opaleye, transformers)
    After searching the rest of the dependency tree exhaustively, these were the
    goals I've had most trouble fulfilling: transformers, contravariant, opaleye,
    base, dotenv, opaleye:test

After:

    Resolving dependencies...
    cabal: Could not resolve dependencies:
    [__0] trying: opaleye-0.6.7003.0 (user goal)
    [__1] trying: transformers-0.5.5.0/installed-0.5... (dependency of opaleye)
    [__2] next goal: contravariant (dependency of opaleye)
    [__2] rejecting: contravariant-1.5 (conflict: opaleye => contravariant>=1.2 &&
    <1.5)
    [__2] rejecting: contravariant-1.4.1, contravariant-1.4 (conflict:
    transformers => base==4.12.0.0/installed-4.1..., contravariant => base<4.12)
    [__2] rejecting: contravariant-1.3.3, contravariant-1.3.2,
    contravariant-1.3.1.1, contravariant-1.3.1, contravariant-1.3,
    contravariant-1.2.2.1, contravariant-1.2.2, contravariant-1.2.1,
    contravariant-1.2.0.1, contravariant-1.2 (conflict:
    transformers==0.5.5.0/installed-0.5..., contravariant => transformers>=0.2 &&
    <0.5)
    [__2] rejecting: contravariant-1.1, contravariant-1.0, contravariant-0.6.1.1,
    contravariant-0.6.1, contravariant-0.6, contravariant-0.5.2,
    contravariant-0.5.1, contravariant-0.5, contravariant-0.4.4,
    contravariant-0.4.3, contravariant-0.4.1, contravariant-0.4,
    contravariant-0.3, contravariant-0.2.0.2, contravariant-0.2.0.1,
    contravariant-0.2, contravariant-0.1.3, contravariant-0.1.2.1,
    contravariant-0.1.2, contravariant-0.1.1, contravariant-0.1.0.1,
    contravariant-0.1.0 (conflict: opaleye => contravariant>=1.2 && <1.5)
    [__2] fail (backjumping, conflict set: contravariant, opaleye, transformers)
    After searching the rest of the dependency tree exhaustively, these were the
    goals I've had most trouble fulfilling: base, opaleye, contravariant,
    transformers

In this case, the feature found a minimal conflict set, {base, opaleye,
contravariant, transformers}, by removing dotenv and opaleye:test from the
original conflict set.

RetryLog is simpler and more efficient than Progress for continuing the solver
log after an error.

This commit generates the solver's final error message in a separate function,
which is more flexible when the solver is run multiple times.

This flag changes the exit behaviour of the v2-test command when the
TargetProblem is that no tests were found. By default, this will result in
'cabal v2-test' returning 1, with the flag enabled it now prints a
friendlier error message and returns 0.

Herbert Valerio Riedel authored 6 years ago and Mikhail Glushenkov committed 6 years ago

cabal-install-2.4 doesn't build anymore w/ GHC 7.8 and older;
so the metadata revision took care to fixup the lower bound of
base and other boot libs to take the versions bundled w/ GHC 7.10.1
as the lower bounds.

Moreover, this also fixes some upper bounds not being PVP compliant.

(cherry picked from commit 0ba16f5c)

- Distribution.Compat.ReadP to Distribution.Deprecated.ReadP
- Distribution.Text to Distribution.Deprecated.Text
- all Text instances needed by cabal-install to Deprecated.Text too
- Distribution.ParseUtils to Distribution.Deprecated.ParseUtils
- Remove deprecated Distribution.PrettyUtils
- new Distribution.Text with

    display = prettyShow
    simpleParse = simpleParsec

  to not break too much stuff (Custom Setup.hs)
- parseInstalledPackageInfo type signature changed to use
  `base` types

This removes around 2k lines from Cabal the library.
git diff --stat shows less, as files are moved (git is smart).
Even so, total 300 lines removal at this point.

Empty and non-empty lines

```
Errors encountered when parsing cabal file ./tree-diff.cabal:

tree-diff.cabal:102:7:
unexpected 'n'
expecting space, "&&", white space, "||", comma or end of input

   98 |       void               >=0.7.2    && <0.8
   99 |
  100 |       -- space and comments
  101 |
  102 |       nats               >=1.1.1    && <1.2,
      |       ^

tree-diff.cabal:86:48: version with tags

   85 |     tagged               >=0.8.5    && <0.9,
   86 |     text                 >=1.2.2.2  && <1.3-foo,
      |                                                ^

```

Since nonInteractive is set in the getSimpleProject step we don't need to
re-check when selecting the main file.

- Separate the logic into own module
- Use same function in `cabal check` codepath
- Tweak the output a little more

Resolves the non-vcs-related part of #5643

NB: this patch mostly moves around code; use
    `git show --ignore-space-change` to facilitate code-review

(cherry picked from commit c40622d4)