Fix caching issues with iter_exes_in_path().

Review Request #12641 — Created Sept. 25, 2022 and submitted — Latest diff uploaded




iter_exes_in_path() was decorated with @lru_cache in an attempt to
avoid numerous filesystem lookups (particularly in unit tests). This
doesn't actually work, though, as you can't cache a generator. Switching
to pre-populating, caching, and returning a list of items would solve
this, but wouldn't be ideal for the common case of just wanting the
first match.

Instead, this change adds more fine-grained caching within the function.
We now cache the existence of an item per-path. This is built up
incrementally, so if we only ever hit one item in the path before
finding a result, we only need to cache and return that item. If we fail
to find a result from any items in the path, a second attempt will
discover that without any file existence checks.

This avoids sporadic problems with unit test failures.

All unit tests pass.