PoC: If import

- Warn about missplaced import (on top level)
- Warn in trees

Remove fields in cabal spec 3.0

- 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`.

Add Distribution.Pretty.Field

- and it's little cousin fromParsecFields,
- Also test that `showFields . fromParsecFields . readFields` roundtrips
  on Hackage Corpus

Something to do: Go from Parsec.Field to Pretty.Field directly,
that's may be useful for some tools

Fix Incorrect `tar.gz` Extension

Refactor options fields to use PerCompilerFlavor type

Remove support for `.zip` format source distributions

Add 'SimplePackage' package type for `cabal init`

E.g. jaeger-flamegraph.cabal failed to roundtrip 'pretty . parse';

Making the list business work didn't feel right,
so I made a bit bigger refactor.

Visibility field

- 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.

Move remainders of ReadP stuff to cabal-install Distribution.Deprecated namespace

- Distribution.Compat.ReadP to Distribution.Deprecated.ReadP
- Distribution.Text to Distribution.Deprecated.Text
- all Text instances needed by cabal-install to Deprecated.Text too
- Distribution.ParseUtils to Distribution.Deprecated.ParseUtils
- Remove deprecated Distribution.PrettyUtils
- new Distribution.Text with

    display = prettyShow
    simpleParse = simpleParsec

  to not break too much stuff (Custom Setup.hs)
- parseInstalledPackageInfo type signature changed to use
  `base` types

This removes around 2k lines from Cabal the library.
git diff --stat shows less, as files are moved (git is smart).
Even so, total 300 lines removal at this point.

Make CabalSpecVersion complete

Fancy error printing

Remove Text from D.BackPack

Empty and non-empty lines

```
Errors encountered when parsing cabal file ./tree-diff.cabal:

tree-diff.cabal:102:7:
unexpected 'n'
expecting space, "&&", white space, "||", comma or end of input

   98 |       void               >=0.7.2    && <0.8
   99 |
  100 |       -- space and comments
  101 |
  102 |       nats               >=1.1.1    && <1.2,
      |       ^

tree-diff.cabal:86:48: version with tags

   85 |     tagged               >=0.8.5    && <0.9,
   86 |     text                 >=1.2.2.2  && <1.3-foo,
      |                                                ^

```