Fix #6879: Use SPDX License list 3.9 for cabal-version: 3.4

Ghc 8.12 features

Fix #6945 Fetch by GITHUB_SHA

Fix ghci being launched before other sources are built

update ghc trac links to point to ghc gitlab

Issue 6807 listbin

This is partial solution to #6807, because we invoke solver.
The follow-up will be to modify Rebuild monad so,
that it can be run without rebuilding (i.e. fail if cache is cold).

Also output extension for executables.

Remove new- command from help (there are v2-)

Add stderr Verbosity modifier

Rework verbosity parsing too and add tests.

Fix typos and language issues

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.

Resolve #6288

- 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

Add check tests for default-language (#6288)

Issue 5586 srcpkgrepo not local

packageDirToSdist

Also calculate hashes for all locally available tarballs.

Rename fields in SourcePackage

Bump bounds in dev-scripts

Also remove specific field for PackageId, as it is in GPD.

Resolve #6281: Add foo:bar syntax to mixins

Fix #6083: allow depending on public sublibrary with pkg:lib qualified syntax

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

Add Pretty Component instance