This change extracts the logic for writing 'requires.txt' from
the ``egg_info`` module into ``_requirestxt``.

It also adds the missing pre-processing for ``install_requires`` and
``extras_require`` previously done in ``dist`` but now implemented in
``_requires.txt``.

For the time being, when `setuptools.build_meta` is called,
`egg_info.egg_base` is accidentally set to the project root between the
several calls to the different build hooks.

This means that if the hooks are called, they will try to overwrite
`setuptools.egg-info/PKG-INFO`, and for a very short interval of time it
will be an empty file.

Another process may then try to simultaneously use `importlib.metadata`
to list entry-points. However to sort entry-points, `importlib.metadata`
will try to read the `Name` field in the `PKG-INFO/METADATA` files and
will raise an error/warning if that file is empty.

This commit tries to use `os.replace` to avoid having an empty PKG-INFO
while `importlib.metadata` is used.

Here we use a specific SHA that we now is "green" to avoid problems with
a moving HEAD.

(An independent validation library makes the confidence in the code
stronger).