Support rebuilding charms with locked wheelhouse

[johnsca]

In order to support patching a stable charm, we need a way to rebuild a charm but not update the wheelhouse of Python dependencies. If charm-build generates a wheelhouse.lock file with specific revs and SHAs that we picked up during the build, then you could pass that in to --wheelhouse-overrides to rebuild with those exactly. We might even look into pipfile / pipenv which has built-in support for generating those lock files.