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>

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