Allow third-party packages to register new rbt commands.

rbt will now scan for third-party commands that are registered
for the 'rbtools_commands' entrypoint.

Testing done: Ran unit tests, manual testing with a third-party
command provider.

Reviewed at https://reviews.reviewboard.org/r/5046/.
Do some trivial cleanup of the _hgcmd method.

Not all these parameters were used, and the name itself was kind of annoying.
Simple cleanup.

Testing done: Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/5117/