resolve all paths before expressing target relative to root

[Gautzilla]

There was a bug where the absolute_to_relative() helper function didn't work with symlinks:

absolute_to_relative(
target_path = r"//ensieta.ecole/recherche/acoustock",
root_path = r"L:/acoustock"
)

where L is a symlink for //ensieta.ecole/recherche.

This raised the following error:

ValueError: path is on mount '\\\\ensieta.ecole\\recherche', start on mount 'L:'

This PR solves the error, and correctly returns the relative path despite symlinks.