When two target names are the same, `mappend`ing them should not
error but just pick the first name.

* Add test for haskell#7423

i.e. Do not warn on -O2 if under off-by-default package configuration
flag conditional.

* Add a regression for:

    * Add another -WErrr test
        This is to make sure we do *not* report it if it is under
        a user, off-by-default flag.
    * Add test for non manual user flags.
    * Add “absolute path in extra-lib-dirs” test
    * Add if/else test
    * Add “dircheck on abspath” check
    * Add Package version internal test
    * Add PackageVersionsStraddle test

* Integrate Artem’s review

(review) Clarify `combineNames` documentation

By explaining the way it operates (working if the two names are equal
or one is empty) and renaming the function from `combineName` to
`combineNames`.

(review) Use guards instead of if/then/else

(review) Match inside argument list

(review) Replace “white” with “allow”

(review) Fix typo in comment

(review) Fix typo in Check module documentation

(review) Harmonise indentation for `data` decls

First field goes in a new line than the data constructor, so we
have more space.

(review) Rename `Prim` module to `Types`

(review) Add checkPackageFilesGPD

`checkPackageFiles` — which works on PD — was used to perform IO. We
introduce a function that does the same thing but works on GPD (which
is more principled).

`checkPackageFiles` cannot just be removed, since it is part of the
interface of Distribution.PackageDescription.Check. Deprecation can
be planned once “new check” is up and running.

* Integrate Andreas’ review

(review) Add named section to missing upper bound check

“miss upper bound” checks will now list target type and name (“On
executable 'myexe', these packages miss upper bounds”) for easier
fixing by the user.

(review) remove `cabal gen-bounds` suggestion

Reasonable as `cabal gen-bounds` is stricter than `cabal check`, see
haskell#8427 (comment)
Once `gen-bounds` behaves in line with `check` we can readd the
suggestion.

(review) Do not warn on shared bounds

When a target which depends on an internal library shares some
dependencies with the latter, do not warn on upper bounds.

An example is clearer

    library
     build-depends: text < 5
    ⁝
     build-depends: myPackage,        ← no warning, internal
                    text,             ← no warning, shared bound
                    monadacme         ← warning!

* Integrate Artem’s review /II

(review) Split Check.hs

Check.hs has been split in multiple file, each une sub 1000 lines:

Check              857 lines
Check.Common       147 lines
Check.Conditional  204 lines
Check.Monad        352 lines
Check.Paths        387 lines
Check.Target       765 lines
Check.Warning      865 lines

Migration guide:
- Check              GPD/PD checks plus work-tree checks.
- Check.Common       common types and functions that are
                     *not* part of monadic checking setup.
- Check.Conditional  checks on CondTree and related matter
                     (variables, duplicate modules).
- Check.Monad        Backbone of the checks, monadic inter-
                     face and related functions.
- Check.Paths        Checks on files, directories, globs.
- Check.Target       Checks on realised targets (libraries,
                     executables, benchmarks, testsuites).
- Check.Warning      Datatypes and strings for warnings
                     and severities.

(review) remove useless section header

(review) Fix typo

(review) Add warnings documentation (list)

For each warning, we document constructor/brief description
in the manual.  This might not be much useful as not but it
will come handy when introducing `--ignore=WARN` and similar
flags.

* (review Andreas) Clarify CheckExplanation comment

Whoever modifies `CheckExplanation` data constructors needs to be
aware that the documentation in  doc/cabal-commands.rst  has to be
updated too.

No need to expose Distribution.PackageDescription.Check.*
to the world. API for checking, for cabal-install and other
tools, should be in Distribution.PackageDescription.Check.

Cabal codebase has now a formatter/style standard (see haskell#8950).

“Ravioli ravioli, give me the formuoli”

See haskell#8963 for reason and clarification requests.

It is now in the Reader part of CheckM monad.

Internal: testsuite, benchmark.
See haskell#8361.

When checking internal version ranges, we need to make sure we
are not mistaking a libraries with the same name but from different
packages. See haskell#9132.

neither…nor, completing what done in haskell#9162

Brandon’s review/II.