Optimisations

[mbway]

This PR introduces some optimisations to improve import times (and therefore startup times).

Initially in #7 I requested that the cache is able to be disabled entirely, but with importlib.reload() being an edge case and importlib.invalidate_caches() existing it seems sufficient that the maturin import hook supports this way of clearing the cache to ensure new modules are detected.

Benchmarking

Benchmarking was carried out using the tests/create_benchmark_data.py script. this script creates a scenario close to the worst case, with 100 paths in sys.path each very long nested paths with two pyproject.tomls present on the path back to the root of the filesystem that must be checked.

Results:

These results are run on the slowest storage I have (a USB drive). Each value is the average over 3 runs.
The 'cold cache' runs are after running echo 3 | sudo tee /proc/sys/vm/drop_caches

	cold cache	warm cache
main	8.80s	6.77s
is_maybe_maturin_project optimisation	7.36s	5.53s
lru_cache	1.95s	0.62s
find_dist_info_path optimisation	1.69s	0.53s

(note that for a more reasonable case such as 20 paths in sys.path, the time taken is ~0.01s)