Allow hackage packages to be target packages

[mgsloan]

Something like:

packages:
- location:
    hackage: xmonad

This would install the version xmonad that's in the resolver. Packages that are not in the resolver would require an explicit version number. This is useful when people want to have a stack configuration which installs some package by default. Perhaps some executable they depend on, or some package they want to package up in a docker image. The main motivating case right now is that some folks want to use stack kinda like a package manager - #1610 #1899 .

We may want to address this cluster of issues first - #1631 #1265 #849 . As the packages vs extra-deps distinction and behaviors is still a bit confusing and doesn't behave as consistently as we'd like with ghc-options and flags. Having hackage packages directly in the packages list could confuse the distinction more. It'd be best to clean this stuff up before extending it further.

[mgsloan]

I think this is adequately covered by either unpacking the package or using a repo, closing.