…info

Changes:

- Deprecate the --egg-base parameter for dist_info and add --output-dir as
  replacement

  (Since the egg format is mostly deprecated, it is nice to move away from
  this nomenclature...)

Avoid using the editables dependency
  - The current implementation is using it to simply place .pth file
    pointing to the project directory anyway...
  - Adding a dependency for creating a file with a single line is a bit
    overkill.

Avoid importing pkg_resources directly
- Setuptools wants to move away from pkg_resources

Replace custom wheel build with re-use of bdist_wheel
  - pro: avoid re-implementing the archiving logic and make sure it is
    compatible with the same archive format used by the final wheels.
  - con: the API of wheel is not exactly stable or exported as public.

This change is heavily inspired by the existing develop command

- Use namespaces.Installer to ensure legacy namespaces are handled.

- Improve the skeleton that would allow the separation between different
  editable strategies
- Extract strategy for static .pth files
- Recognise the static .pth file as the primary strategy for src-layout
  packages in a "non"-strict scenario

- Move test_editable_prefix from test_develop to test_editable_install
- Add more tests for editable install focusing on packages using
  namespaces (legacy or PEP 420)

- Add improvements and small fixes for the static .pth file strategy
- Add initial implementation of an editable strategy based on
  MetaPathFinder but just for the top-level packages/modules
  - The idea here is to create a "loose/lax" mechanism that is able to
    handle the complex mapping mechanisms supported by setuptools (via
    package_dir) while still allowing new modules and files to be added to
    existing packages.

This workaround can be reverted when issue 3261 is fixed.

- Add implementation of editable strategy based on a link tree
  - This is the `strict` implementation.
  - Only files are linked, not directories.
    - This approach makes it possible to use harlinks when softlinks are not
      available (e.g. Windows)
    - It also guarantees files that would not be part of the final wheel are
      not available in the editable install.
- Add non-editable files to the produced wheel wheel (e.g. `headers`,
  `scripts`, `data`)

According to the PEP 420, namespace packages need to gracefully handle
later additions to path.

- Use a `PathEntryFinder` + an arbitrary placeholder entry on `sys.path`
  to force `PathFinder` to create a namespace spec.
  - Since `_NamespacePath` and `_NamespaceLoader` are private classes (or
    just documented for comparison purposes), there is no other way to
    implement this behaviour directly [^1].

[^1]: Reimplementing _NamespacePath + a custom logic to maintain
      namespace portions don't have a corresponding path entry also
      seems to have the same end result.