gh-139899: Introduce MetaPathFinder.discover and PathEntryFinder.discover (#139900)

gh-139899: Introduce MetaPathFinder.discover and PathEntryFinder.discover (#139900)

* gh-139899: Introduce MetaPathFinder.discover and PathEntryFinder.discover

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Fix doc reference

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Remove specific doc references

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Fix docstrings

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Revert "Remove specific doc references"

This reverts commit 31d1a8f5510e0f7a53016c7120ea2e1bda46e60c.

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Fix news references

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Add docs warning

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Raise ValueError on invalid parent

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Dedupe __path__ in PathFinder.discover

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Use context manager and add error handling to os.scandir

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Raise ValueError on invalid parent

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Dedupe when package exists with multiple suffixes

Signed-off-by: Filipe Laíns <lains@riseup.net>
* Apply suggestions from code review

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>
* Add tests

Signed-off-by: Filipe Laíns <lains@riseup.net>
---------

Signed-off-by: Filipe Laíns <lains@riseup.net>
Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>
Co-authored-by: Brett Cannon <brett@python.org>