Support for multiple/non-standard package indices

[snoyberg]

One thing cabal did right was support for multiple package indices (remote-repos). I think we should probably do the same thing. One thing cabal did badly was how multiple repos get coalesced from multiple config files. We can do better. Here's a proposal:

• Each repo/index gets a unique identifier (e.g., hackage.haskell.org)
• Each repo can have Git and HTTP URLs configured for it
• We follow normal overriding rules for Stack.Config
• We also support the notion that if neither a Git nor HTTP URL is specified, the repo is disabled (we could also be more explicit with disabled: true). That would allow you, for example, to disable usage of hackage on a per-project basis

We need to ensure that tarballs are downloaded to repo-specific directories (like cabal does), since hackage's foo-1.2.3 may very well be different than some other repo's foo-1.2.3.

[snoyberg]

I'm working on this, and it's a fairly large (albeit mechanical) change.