Tools works with groups

Make Nit tools more POLA when projects (or groups) are given on the command line.

eg. people using `nitunit ../lib/mylib` may expect that all the unit tests of the lib are executed, not just those from the default module `../lib/mylib/mylib.nit` (that often is basically an empty bottom module)

So this PR introduces `parse_full` in the loader that loads and returns all the modules of given groups, then adapt most tools to use `parse_full`.

No more crazy shell pipelines with `nitls` or `find` to use nitunit, nitpick and other on all the modules of your libs or programs.