rebase vs rewrite (and renaming plugins in general)

[Ian Clatworthy]

Hi Jelmer,

The rebase plugin now is called rewrite but the change looks like it's
skin-deep, i.e. at the python package level, things still live under
bzrlib.plugins.rebase. I don't think that's what we want as it will lead
to confusion and break stuff (like documentation generation).

I think we ought to put some policies in place wrt renaming plugins,
e.g. leave the old plugin there, start a fresh project and point the old
project to the new project. That seems the right balance to me between
retaining backwards compatibility and forward internal consistency.

What do you think?

Ian C.