We have a lot of support code in rbtools.clients.__init__, with more

coming. This has been hard to maintain, and was going to create some

circular dependency issues.
This cleanup has been long-overdue. This change moves SCMClient,

RepositoryInfo, PatchAuthor, and PatchResult into sub-modules of

rbtools.clients.base.
In the process, SCMClient has been renamed to BaseSCMClient, to

better fit modern naming of base classes.
There is a legacy SCMClient class in rbtools.clients that emits a

deprecation warning when instantiated. It will be removed in 5.0.
Aside from that, all the changes are mechanical or updates to references

in documentation.

All unit tests pass.
Posted this change using this build.