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This commit also refactors the Dependencies type so that it can represent any
combination of dependencies, buildability, and visibility.

This commit tracks dependencies on sub-libraries by extending the functionality
for tracking executables that was added in
e86f8389.

It also starts adding support for library visibility, though it currently only
works for source packages.  There is a TODO for handling installed packages.

This commit handles visibility similarly to the way that the buildable field is
handled currently.  It only checks whether a component is made private by the
current environment and flag constraints at the start of dependency solving.
This means that the solver can treat a component as visible when the visibility
is controlled by an automatic flag, and the build can fail later, depending on
the value that is chosen for the flag.

Fixes #6038.

Fix Windows GitHub Action

- msys2 should be installed
- there is cabal and GHC, but we want to be sure about
  versions, so we reinstall.
- Reinstalling GHC doesnt' "just work", so we uninstall it first.
  This takes time, but makes CI work.

Update dependency bounds

- Also drop GenericInstances as URI has Generic instance now.
- simplifier goes nuts on ProjectConfig instances...

Prepend hs-source-dir to match-component

Add Integration tests for File Target Selector

Add Integration test for Target Selecetors for files that
are part of other-modules in sub-directories.

Normalise Filepaths for target matching

Add filepath and module target syntax to readthedocs

Add ambiguous file target test-case

Refactor shared TargetProblem data types into their own module.

Moved "problem rendering" to CmdErrorMessages module

Additions by Oleg Grenrus:

- There were CommonTargetProblem, but now
  TargetProblem has an extension point so we can have just one type.
  A lot of code is simplified as we don't need to pass in injection
  from CommonTargetProblem to the resulting `err` type.

Make cabal-install compilable with NoImplicitPrelude

I.e. find out where we don't yet
used `Distribution.Client.Compat.Prelude`.

- If the module is small I added direct `Prelude` imports.
- Add Exception, deepseq stuff to Cabal Prelude
- Add Parsec, Pretty and Verbosity to Client Prelude
- use for, for_, traverse and traverse_ (removes need for Control.Monad)

Make own modules for InstallMethod and OverwritePolicy

Issue 6809: v2-update -z

This essentially makes -z/--ignore-project appear in all v2-commands

Resolve #6804: v2-update reports new index-state

Fix #6805: More compatible rpmvercmp behavior

Regex list

For example:

    Downloading the latest package list from hackage.haskell.org
    Updated package list of hackage.haskell.org to index-state 2020-05-15T09:13:06Z
    To revert to previous state run:
        cabal v2-update 'hackage.haskell.org,2020-05-10T18:58:02Z'

Or

    Package list of hackage.haskell.org is up to date at index-state 2020-05-15T09:13:06Z

To get exact match you can

    % cabal list ^QuickCheck$ -I
    * QuickCheck
        Synopsis: Automatic testing of Haskell programs
    ...

But not the prefix/suffix/regexp matching is in your power for
searching. e.g.

    % cabal list ^Cabal
    * Cabal
        Synopsis: A framework for packaging Haskell software
    ...
    * cabal-install
        Synopsis: The command-line interface for Cabal and Hackage.
    ...
    and many others

Resolve #6409: Add public multilib test(s)

    % cabal list transformers --installed
    * transformers
        Synopsis: Concrete functor and monad transformers
        Default available version: 0.5.6.2
        Installed versions: 0.5.6.2
        License:  BSD3

    % cabal list transformers --installed -w ghc-8.10.1
    * transformers
        Synopsis: Concrete functor and monad transformers
        Default available version: 0.5.6.2
        Installed versions: 0.5.6.2
        License:  BSD3

    % cabal list transformers --installed -w ghc-7.6.3
    No matches found.

    % cabal list transformers --installed -w ghc-7.8.4
    * transformers
        Synopsis: Concrete functor and monad transformers
        Default available version: 0.5.6.2
        Installed versions: 0.3.0.0
        License:  BSD3

Also add a test for current behaviour
of https://github.com/haskell/cabal/issues/6083

And variant with mixin, for https://github.com/haskell/cabal/issues/6281

The tests are disable for GHC older than 8.8
Should they work?

Add NonEmptySet and use it in Dependency

Described Mixin

Remove Text type-class