Per-component dependency solving

I don't think there is a clear design for component-based solving yet, so I wanted to create an issue where we can discuss it. #1575 addresses circular dependencies in test suites but is moving towards a different solution.

Related issues:
#779, #3978, https://github.com/haskell/cabal/issues/3492#issuecomment-225391004 - solver doesn't reject configurations that require unbuildable/non-existent components
#1575 - cabal can't handle cycles between packages that are not cycles between components
#2725, #5413 - cabal requires dependencies of components that aren't being built
#3662 - more per-component support
#3263 - fine-grained dependencies
https://github.com/haskell/cabal/issues/6039

Update: The main design is in https://github.com/haskell/cabal/issues/4087#issuecomment-287665861.