Support packages pulled from VCS

[Ford_Prefect]

It'd be nice to have a way to just pull a package from VCS. Can just have a set of mappings of upstream-vcs-revision -> our-package-revision.

Relevant log for reference:

17:02 < ferringb> should pull the etemplate code we use at work for this
17:03 < ferringb> can just do a custom repository type for it that has a pmaint 
                  <action> addition
17:03 < ferringb> secondary bug...
17:03 < ferringb> need to be able to extend the pmaint actions dependant upon 
                  repository/plugins
17:03 < Ford_Prefect> You can just have upstream-tag-or-id -> revision mppings
17:03 < ferringb> yep
17:04 < ferringb> original approach I'd thought about was basically just <atom> 
                  <rev1> <rev3> <rev44500>, etc, in a file to control the 
                  revnos/versions exposed in the fake repo
17:04 < Ford_Prefect> Want upstream's actual release version in there too
17:05 < ferringb> personally, yes
17:05 < ferringb> although a pmaint action to add a revno to the crappy text 
                  database I mentioned wouldn't be a bad thing