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Previously the user's guide stated that the extensions field was
preferred over ghc-options, even though extensions are deprecated in
favour for default-extensions. This commit updates the reference to
default-extensions instead.

While those buildinfo fields were added to the parser some time
ago via 57d7f28e and
4a287659 that work was never completed
by implementing the necessary build/sdist logic in Cabal.

This commit remedies this oversight by implementing and wiring up the
missing build logic.

*WARNING* this commit is still very WIP; ASM works mostly; C--
          support is still incomplete

* Add deprecated hs-source-dir

I replaced the note about `hs-source-dir` being deprecated in the
description of `hs-source-dirs` by a field of its own to include the
`deprecated` and `removed` fields because it's no longer in 3.0.

* Add note to hs-source-dirs about multiple components

Source directories can be shared among components but there's a
non-obvious caveat which I've tried to clarify with a note.

* Clarify shared source dir note

Modules in a shared source directory can inadvertently end up getting
compiled multiple times.

[ci skip]

Fixes #5986.

autogen-includes aren't searched nor packages by `sdist`.

This is relatively small code patch, but there are
- change in file-format
- short documentation of the field
- `cabal check`
- test-suite noise due new field in `BuildInfo`

Starting with `cabal-version: 3.0` free text fields
preserve indentation and blank lines.
In other words, we don't need single dots to indicate blank line.

IPI is not versioned. So we default to latest variant always.
This is little harm, as old GHC use old Cabal to print
IPIs. And free text fields are only informative.

Fixes #5938

  build-depends: network ^>= { 2.6.3.6, 2.7.0.2, 2.8.0.0, 3.0.1.0 }

[ci skip]

[ci skip]

- extensions
- (singular) hs-source-dir
- build-tools

Example, with `cabal-version: 2.5`

```
[laptop] tree-diff % cabal new-build --dry
Errors encountered when parsing cabal file ./tree-diff.cabal:

tree-diff.cabal:59:3: error:
The field "extensions" is removed in the Cabal specification version 2.5. Please use 'default-extensions' or 'other-extensions' fields.

   58 | library
   59 |   extensions: CPP
      |   ^

tree-diff.cabal:60:3: error:
The field "extensions" is removed in the Cabal specification version 2.5. Please use 'default-extensions' or 'other-extensions' fields.

   59 |   extensions: CPP
   60 |   extensions: DeriveGeneric
      |   ^
```

I needed to add new CabalSpecVersion to properly issue deprecation
messages of `extensions`.

- for main library default is True
- for named libraries default is False
- remove Monoid Library instance; i'd rather remove emptyLibrary too
  but now these (two?) usages are explicit:
  - creating Library from BuildInfo when merging common stanzas
  - creation of dummy package in `cabal new-repl`
- Documentation is left for Francesco to write
- Field is named `visible` because we have `buildable` and `exposed`
  adjectives. `visibility: True` is unclear.

Fix a typo

Clarify pkg-config (closes #4242)

Clarify cabal-version (closes #2571)

Minor tweak to description of extra-bundled-libraries.

cc @phadej

and clean up various aspects related to the `cabal-version` field

[skip ci]

Also improve the wording slightly about the `--new-freeze-file`
requirement.

[skip ci]

Now the behaviour doesn't switch on just --project-file's
presence.

This doesn't get anywhere near the improvements suggested in #4831,
but it's a very respectable improvement over the status-quo for not
much effort.

On all platforms, warn about their presence. On Windows, we should use
slashes (as opposed to backslashes) where possible, to avoid causing
things like #5386.

Closes #5386.

It mentions the issue of parsing failures caused by the presence of
blank spaces after opening parentheses.

See #5150, #5292, #5293.