Tweak the typing spec's module resolution to more closely emulate Python's runtime semantics

[AlexWaygood]

This PR implements the changes to the typing spec's module resolution order that have been proposed on Discourse. Three changes have been made:

1. Typeshed’s standard-library stubs have been moved higher in the resolution order, above any items that originate from site-packages.
2. With the change from point (1), it becomes more important that type checkers provide a clear and easy way for users to override the vendored copy of typeshed’s standard-library stubs with a custom directory of standard-library stubs if they want to. Most major type checkers already implement this (mypy provides the --custom-typeshed-dir option; pyright provides the typeshedPath configuration-file option; pyre provides the --typeshed option). This update to the spec formally specifies that doing so is highly encouraged.
3. The PR brings back an explicit mention in the typing spec of where vendored typeshed stubs for third-party packages (if there are any that the type checker has chosen to vendor) should come in the module resolution order (last!). This was specified in PEP 561, but the relevant language was removed from the typing spec in [spec] Update typeshed language to conform to reality #1571.

Please see the Discourse topic for a detailed rationale for these changes, and please post any substantive feedback on the proposed changes there rather than on this PR. (Suggestions for minor wording improvements are welcome here.)

As discussed on Discourse, the proposed new specification appears to already be implemented by mypy, but pyright's behaviour would have to change to become conformant with the new spec. (I have not surveyed pyre's and pytype's module resolution behaviour.)

[emmatyping]

As the original PEP author I like this change. It better reflects the current ecosystem of type checkers.

[carljm]

Typing council members have all signed off on this. @AlexWaygood, ready for merge?

[AlexWaygood]

I was waiting for at least a week after filing the Typing Council issue, as per the process described at https://github.com/python/typing-council?tab=readme-ov-file#decisions. But as of this morning, it is exactly 7 days since I posted that issue, so perhaps I'm good to go now!