Don't require packages to be from hackage.haskell.org

Instead of requiring that packages come from the 'main' hackage server,
allow them to come from any mirror.
hlint sugestions for RpmBuild.hs
Merge git://github.com/CodeBlock/cabal-rpm

Conflicts:
	src/Commands/RpmBuild.hs
RpmBuild.hs: rework/simplify rpmBuild, change existsInCache to copyCachedTarball

- improve logic so it checks for tarball in cwd
- give up if none and .cabal file is in SCM dir

One edge issue: cabal fetch could fail despite having the tarball
cached already from another remote repo (mirror).
It would be nice if cabal supported writing a tarball to a specific dir.