Fix bug in list_file_walk when cwd != path

Previously this was calling .is_file() on a relative path.  This would fail if
the path was not relative to the current working directory, for example when
fingerprinting a path dependency.
Test for build command fingerprints in path deps

This tests the fix for list_files_walk from #561.
Fix tests on Mac builders

Comparing URIs in /tmp was failing because of symlinks.
Fix list_files_git for paths in subdirs of a repo

list_files_git previously assumed that the package was always at the root of
the git repository.  This broke when trying to list files for a package in a
subdirectory of a repo, or in other cases, e.g. if the user's home directory
is a git repo.