path-util: rework file_in_same_dir() on top of path_extract_directory()

path-util: rework file_in_same_dir() on top of path_extract_directory()

Let's port one more over.

Note that this changes behaviour of file_in_same_dir() in some regards.
Specifically, a trailing slash of the input path will be treated
differently: previously we'd operate below that dir then, instead of the
parent. I think that makes little sense however, and I think the code
using this function doesn't expect that either.

Moroever, addresses some corner cases if the path is specified as "/" or
".", i.e. where e cannot extract a parent. These will now be treated as
error, which I think is much cleaner.