update PYTHONPATH when running from source

[Jelmer Vernooij]

After moving to main.rs, when brz is run from a source directory we no longer include the source directory on PYTHONPATH by default.

This is a little annoying, since it means you always have to manually add it.

We should update sys.path to start with the brz executable's directory when brz is run from a directory that contains a "breezy" subdirectory. Alternatively, maybe we can just load the breezy module and set sys.modules manually?