[FIX] tools.translate: addons path resolution failed with duplicates

When the addons_path config contained the
default path, the system was working with
an empty addons_path in the middle of the
other paths. This empty one matched for all
files, making all files appear to belong to
the root path component, e.g. `home` if the
root path is in /home.
Refactored a bit to avoid duplicates and
redundant path calculations.