Fix -x option behavior

[rindeal]

This PR addresses an issue where target_dirs were being optimized out before being passed to the get_filesystem_devices() function. The changes in this PR ensure that target_dirs are passed directly to get_filesystem_devices(), and only then are they simplified.

Example of a command that currently doesn't work correctly:

dust -x $(findmnt -S tmpfs -o target -n)

It should show the usage of all tmpfs mounts, but it currently shows just the the root mountpoints like /run, since all the mountpoints nested under it are optimized out.

[bootandy]

Nice find, good edge case.

[bootandy]

Fix the lint and I'll accept - thanks

[rindeal]

Done.

Also I streamlined the funcs so they all accept AsRef<Path>.

When running CI I noticed some tests are randomly failing, so I fixed that as well.

Hope you don't mind the new changes.

[bootandy]

Thanks,

Agree with the streamlining.

Those tests, were new tests and I refactored them here: #404 - so we no longer need that part of your PR. - which now causes a conflict :-(.

(Although I do like your use of $(mktemp -d) so feel free to keep that if you'd like).

[rindeal]

Conflicts from those tests would be fine, but there were many complex conflicts from #399

mktemp -d would require needless code verbosity, your once initialization looked better to me, I just changed it to native code and to use a global constant to make it more robust.