Tolerate concurrent sys.path_importer_cache races in the loader (#61734)

[ggiesen]

What does this PR do?

Makes the module loader tolerate a shared-cache race that fails module loads on
a busy deltaproxy.

When several loaders run in one process and load modules concurrently -- e.g. a
deltaproxy hosting many sub-proxy minions -- they share the global
sys.path_importer_cache. The loader's __clean_sys_path runs after each
module load and calls importlib.invalidate_caches(), which walks that cache
and deletes stale entries. If another loader already removed an entry that
CPython is walking, invalidate_caches() raises KeyError (or RuntimeError
if the mapping changed size mid-iteration). That exception propagated up through
_load_module and failed the module function that happened to trigger the
cleanup, e.g.:

File ".../salt/loader/lazy.py", line ..., in __clean_sys_path
    importlib.invalidate_caches()
File ".../importlib/__init__.py", line 71, in invalidate_caches
    finder.invalidate_caches()
File "<frozen importlib._bootstrap_external>", line ..., in invalidate_caches
KeyError: '/usr/local/lib/python37.zip'

The fix wraps invalidate_caches() so the race is tolerated -- the entry was
going to be invalidated regardless -- and drops stale FileFinder entries from
sys.path_importer_cache with pop() instead of del, so the same concurrent
removal cannot raise between the presence check and the delete.

What issues does this PR fix or reference?

Fixes #61734

Previous Behavior

On a deltaproxy running many sub-proxy minions, a scheduled/custom module call
would intermittently fail with KeyError from importlib.invalidate_caches.
The reporter notes it scales with the number of sub-proxies per deltaproxy and
that catching the error in the loader resolves it.

New Behavior

The concurrent sys.path_importer_cache race is tolerated; the module load
completes.

Merge requirements satisfied?

• Docs
• Changelog
• Tests written/updated

tests/pytests/unit/loader/test_lazy.py::test_clean_sys_path_tolerates_shared_cache_race
patches importlib.invalidate_caches to raise KeyError/RuntimeError and
asserts __clean_sys_path does not propagate it. It fails against the current
code. (The race itself is timing-dependent and hard to reproduce
deterministically -- matching the report -- so the test exercises the tolerance
directly.)

Independent of #69787 (both touch salt/loader/lazy.py but in different
functions -- __populate_sys_path/_load_module there vs __clean_sys_path
here -- so they merge cleanly).

Commits signed with GPG?

No