Update _frozen_importlib_external for Python 3.15

[raul-sq]

Syncs _frozen_importlib_external with Python 3.15 changes and removes the
eight corresponding TODO entries from
stdlib/@tests/stubtest_allowlists/py315.txt:

• load_module() removed on _LoaderBasics and NamespaceLoader
  (deprecated since 3.4 and 3.10 respectively): declarations gated with
  sys.version_info < (3, 15) plus @deprecated, following the existing
  find_module pattern in these stubs.
• cache_from_source(): the debug_override parameter (deprecated since
  3.5) was removed in 3.15; the function is now version-branched, keeping the
  previous overloads unchanged for <= 3.14.
• SourceLoader.source_to_code() / SourceFileLoader.source_to_code():
  Python 3.15 adds a fullname parameter; signatures are version-branched.
• PathFinder.discover() / FileFinder.discover(): new methods in 3.15
  (classmethod and instance method respectively), added under
  sys.version_info >= (3, 15).
• NamespacePath: _NamespacePath was made public in 3.15 (the private
  name is kept as an alias); the class is added under
  sys.version_info >= (3, 15).

The discover() and NamespacePath signatures were derived from
Lib/importlib/_bootstrap_external.py on the CPython 3.15 branch and may
benefit from extra reviewer scrutiny.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉