Don't strip pathname from debug symbol files

When GDB looks for debug symbols it looks for the full path of the file
inside the configured debug-file-directory. For example, if the
debug-file-directory is set to a default of /usr/lib/debug, and you are
debugging /usr/bin/python3, GDB will look for its debug symbols at this
location: /usr/lib/debug/usr/bin/python3.

BuildStream has been putting all debug files inside /usr/lib/debug
under their $(basename), so in the above example GDB would fail to find
any debug symbols for /usr/bin/python3 because they would be in the
incorrect locatoin of /usr/lib/debug/python3.