mkws - Make GTWS_FILES_EXTRA sematics more useful

Before , GTWS_FILES_EXRA was relative to the repo origin itself, and
installed into the same location in the WS repo as the origin repo.
However, this doesn't work well for bare repos.

Entries in GTWS_FILES_EXTRA are now:
- Relative to the project origin, not the repo origin
- Contain : seperated segments, where the first is the origin location,
  and the second is the path in the WS repo to which to install
- If the second element is not given, the file is installed in the same
  location in the WS repo as in the origin repo
- It now works for remote repos