[Distutils] Adding Provides-Extra as an edit to PEP 345

[Daniel Holth]

> Though in the interest of not having too many different ways to do a
> similar thing, could this not also be used for tests/docs?  Perhaps
> even "Provides-Extra: tests", and "Provides-Extra: docs" should be
> reserved names as you suggested.  I envision something like:

...

> Provides-Extra: setup
> Requires-Dist: mysetuphooks; extra == 'setup'
>
> for build-time dependencies.  Although in that case, pysetup (or any
> other installer) would explicitly have to check for this in the
> metadata before doing anything else.  So maybe this example stretches
> this syntax too far?  Plus build-time dependencies are not exactly
> "extras".  It's very much required to do anything with the package.

Pre-built or binary packages won't want to install the build-time
dependencies. The check for a particular Provides-Extra is trivial,
just a list membership test.

I expect to see a lot of bugs in packages that require some of their
build-time dependencies at run time.