Pass through the git_root_dir, if present

Otherwise the file > blame > commit sequence breaks.

Don't shadow the builtin commands module

This is only really relevant for ST2, which doesn't place package contents
into their own module... but requires that all commands be exposed in a top-
level file.

Fixes kemayo#458.

Missing packages prefix

Some windows path errors if os.environ doesn't contain all values

Fix ST3 plugin directory detection

Don't use the async events, because ST2