don't add directory that doesn't include any files to $PATH or $LD_LIBRARY_PATH

[Flamefire]

While we already have detection of those containing any files recursively that is not enough: Those paths are NOT searched recursively, hence we need to check those folders only.

This happens a lot e.g. for Python packages where we have lib/python3.7/site-packages/foo.py with an otherwise empty lib.

And as each path added to those 2 env vars slows down execution/library loading it makes sense to clean those more.

[ocaisa]

LGTM

[Flamefire]

BTW: I'm not sure if we should do the same for LIBRARY_PATH as is done in e.g. PythonPackage, which would make that redundant and allow for removal of that. However for LIBRARY_PATH=/tmp/foo gcc -l:bar/lib.so it is possible to find /tmp/foo/bar/lib.so. This is a VERY unusual use and I'm not even sure if intended, but it is possible...

[ocaisa]

The fact that that is possible is perhaps enough for me to not include LIBRARY_PATH in this PR. LIBRARY_PATH is only used at link time so I don't think it's going to have the same level of impact as the other two.

[ocaisa]

Thanks for the improvement @Flamefire !