Add `discover()` method to (all) finders

[FFY00]

Feature or enhancement

Proposal:

It would be nice to implement discover() for all finders where that can be reasonably achieved.

• BuiltinImporter
• FrozenImporter
• WindowsRegistryFinder
• PathFinder
• FileFinder
• zipimporter

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

[FFY00]

Ideally, this should be done before the 3.15 feature freeze.

[Locked-chess-official]

For FrozenImporter, I found no way to get totally all frozen modules. Maybe it needs a private api to export them.