Easier fallback repos

[sc0ttj]

6f2b899

This change means that when users switc hto these repos, Pkg may not (depending on settings) search any other repos for deps... This is probably want we want for common and noarch repos, but might not be elsewhere...

I propose we add a new feature to Pkg:

• allow all, any or * as a fallback repo in sources-all (etc), so Pkg will fall back to any installed repos it finds
• allow empty field, or none for no fallbacks