Add support for master_repositories

[compnerd]

Copying around exlibs is stupid. We need something like master_repositories to provide a way to use exlibs from other repositories.

[pioto]

hmm... what about having the search path for exlibs be... the current repo, the master repo (if specified), then any other repo with format = exheres, in order of importance.

not sure how much would be involved in that, but i'd assume it would basically involve paludis putting more into the EXLIBSDIRS env var

[ciaranm]

Mmm, that's quite a bit simpler to do, and a lot less messy. Certainly worth considering...