Allow targets to override snapshot versions?

[mgsloan]

Thanks to zedbourbaki in #haskell IRC for bringing this up. If you try to override a package version that's in the resolver, via stack install, it refuses:

mgsloan@computer:~$ stack install hmatrix-0.17.0.1
Run from outside a project, using implicit global config
Using resolver: lts-3.10 from global config file: /home/mgsloan/.stack/global/stack.yaml
Error parsing targets: Specified target version 0.17.0.1 for package hmatrix does not match snapshot version 0.16.1.5

This is a pretty good message, but particularly when using the global project, to me it makes sense to implicitly make this into an extra-dep. Some of the checkboxes on #272 seems to suggest that this was intended at some point:

• To force a specific version of a package to be installed, can run stack install foo-1.2.3
• For ease of use, can provide stack install foo:latest to ignore whatever's in the snapshot

[borsboom]

I agree, seems like that should work based on #272.