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.