Speed up expand_directories function

[mazulo]

In a directory with 1411 valid files, the new implementation using os.scandir took simply 8ms. The previous implementation for the same project would found more files (6692) but most of them not valid ones.

Here are the results using different implementations:

❯ python second_performance_test.py
os.walk                 took 20 ms. Found files: 1411
os.walk-glob    took 30 ms. Found files: 1399
glob.glob               took 26 ms. Found files: 1385
glob.iglob              took 27 ms. Found files: 1385
pathlib.rglob   took 47 ms. Found files: 1406
find_files              took 39 ms. Found files: 1411
fast_scandir    took 8 ms. Found files: 1411
run_previous_os_walk_from_the_project   took 22 ms. Found files: 6692

and here is the code for this: https://gist.github.com/mazulo/f1fe5047fda3ab3a8f965ddfb7e39f2a