Do a proper page/code split of cvs.module

Now that we're ported to D6 (and the dust has mostly settled), we can make use of the D6 menu system to do a proper page split into sane .inc files. I'd like to do this before the 6.x-1.0 release. It's a little too bad to spend effort on this instead of versioncontrol API, but at least for the near future, d.o is going to be running cvs.module, and it'd be nice not to load all 3000 LoC on every page load when the vast bulk of the pages don't care about any of this code at all.

See #390856: Do a proper page/code split of project and #405478: Do a proper page/code split of project_issue for the page splits for the rest of project*.