Modules in rbtools.commands.base now provide all the base command

classes, option processing, output handling, and errors. This cleans up

the rbtools.commands module and gives us room for further expansion.
This work was planned for RBTools 4, but had to slip.
With the exception of the, well, exception classes, all classes use

housekeeping.ClassMovedMixin to point to the new class. We don't

bother subclassing the old exceptions because catching a subclass

wouldn't do any good. We can't easily deprecate those, for the

time-being, we'll leave them in, directly assigning from the new

modules.
The moved classes have not been modernized. Aside from imports and

version change information in the docstrings, no changes have been made

to any of the moved code.

Unit tests pass, except for one that's sensitive to generated output

and is seeing a warning. This will be addressed in a follow-up change.