I introduced TestCode which allows to report more status information
than bare ExitCode.

This looks like an accident from a6e427ac
It causes cases like this to fail:

$ cat foo.c
int foo() { return 42; }
$ cat Lib.hs
module Lib where

foreign import ccall "foo" foo :: Int

bar = foo
$ cat cabal-csrc-repl.cabal
cabal-version:      2.4
name:               cabal-csrc-repl
version:            0.1.0.0
library
    exposed-modules:  Lib
    build-depends:    base ^>=4.14.0.0
    C-sources:        foo.c
    default-language: Haskell2010
$ cabal v2-repl
Resolving dependencies...
Build profile: -w ghc-8.10.1 -O1
In order, the following will be built (use -v for more details):
 - cabal-csrc-repl-0.1.0.0 (lib) (first run)
Configuring library for cabal-csrc-repl-0.1.0.0..
Preprocessing library for cabal-csrc-repl-0.1.0.0..
GHCi, version 8.10.1: https://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Lib              ( Lib.hs, interpreted )
Ok, one module loaded.
*Lib> foo
ghc: ^^ Could not load '_foo', dependency unresolved. See top entry above.

- Add availableSince to language/extension fields
   - Also autogen-modules once we are on that.
- Remove default-language check for cabal-version: 3.4

We cannot make the field `uniqueFieldAlaDef`, as that would require
specifying `default-language` in each subconditional.
I leave it for https://github.com/haskell/cabal/issues/6925

Note: the default language is whatever is the compiler's default.
The GHC-8.10 default is its variant of Haskell2010
with NondecreasingIndentation and without DatatypeContexts

Note: configuredPackageProblems is a mess,
and there might be bugs now.

This is preparation to solve #6083.
As such, this shouldn't affect anything yet.

It's import that flag is *True*, so it's passed without prefix-sign.

GHC <8.8 isn't able to read the "visibility" field from the package
database, since it's built against an older Cabal. For this reason,
using multiple public libraries with it did not work (all sublibraries
appeared as private).

This patch makes cabal-install override Cabal's library visibility
checks when GHC is older than 8.8, extending the multiple libraries
feature compatibiliy to older GHCs.

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?

Removes command and cleanups cabal-testsuite.
The tests for #3199 #4099 #3436 are removed, but they seem to be
sandbox specific issues.

Removes Sandbox.Types, Sandbox.Index and Sandbox.Timestamp modules.
The Sandbox and Sandbox.PackageEnvironment are still
there as some configuration in v1-commands happens through them
(~/.cabal/config vs ./cabal.config).

BuildExFlags contained only sandbox specific parameter,
so it's removed as well.

Remove sandbox support from cabal-testsuite
Remove sandbox from GlobalFlags and Sandbox unit-tests

These constructors don't carry any additional semantics.
They were used in `cabal check` implementation, but now already parser
reports errors so we don't need these redundant constructors anymore.

MajorBoundVersion (^>=) has slightly different semantics:
we want be able to relax only `^>= x.y.z` upper bounds.
Therefore it's preserved.

Also refactor ./Setup sdist to not do anything fancy.
Now `./Setup sdist` and `v2-sdist` should produce close(r)
to same tarballs.

- We had to add gpdScannedVersion to permit solver to report
  about too new packages in the index.
  https://github.com/haskell/cabal/issues/6652
- It's hard to track what happening with Setup interface selection in
  ProjectPlanning.
  https://github.com/haskell/cabal/issues/6651 would help to
  make sense of it.

The cabal command interface will drift more and more from
./Setup interface. This is first step: don't assume
they are the same.

This removes need to "patch" setup commands to work with cabal:
the `act-as-setup` SHOULD behave as (simple) ./Setup

There were few setupAndCabalTests which weren't `build-type: Simple`.
For those I made a separate `cabal.test.hs`.

Also added a OutputNormalizer for global tmp directory.
`v2-build` sdists `build-type: Custom` packages to get the list of files
to watch. I guess it's ok to have that functionality,
yet it could be eventually removed, as `v2-sdist`
does not invoke `./Setup.hs` script.

And make it pipe output to `man -l -`.

Fixes #6533

This commit is the same as 96c3bf91, but with
the unit tests updated to fix #6495.  The tests check less of the solver output
so that they are less fragile.

Also split workflows into parts, helps with restarting.
https://github.community/t5/GitHub-Actions/re-run-jobs/m-p/30476#M440

This reverts commit fbe7d73e, reversing
changes made to f6e7cdd6.