Add runParsecParser', which takes CabalSpecVersion

Use FieldName in FieldGrammar.FieldDescrs

Add showFields-prime

Introduce Distribution.Fields module namespace

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.

Add rules to run tests via "make parser-tests" etc

Add hacked the --cwd argument parsing to
parser-tests and check-tests. It's a kludge
but it's good enough.

Make hackage-tests parallel. Resolves #5736

Solver: Improve error message by finding a minimal conflict set (issue #5647).

Fix `--with-compiler` failing to locate compiler on Windows

pass --host=$HOST to configure when cross-compiling

Make v2-test succeed when no test suites found

Automatically pass -x to hsc2hs when cross-compiling

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.