'cabal freeze' can create a constraint on 'rts'

@grayjay [writes](https://github.com/haskell/cabal/pull/2787/files#r37700715):

> I also found that `cabal freeze` can create a constraint on `rts`, even though `rts` isn't in the source index.

I think that `freeze` shouldn't do that, since `rts` isn't installable and its version doesn't change with GHC versions (though it shouldn't matter anyway).

/cc @benarmston

Start by locating the implementation and tests for the `cabal freeze` command, then trace how resolved packages become constraints and why `rts` is included despite not being in the source index. Done means freeze no longer writes an `rts` constraint while retaining constraints for installable packages; add or update coverage for this case.

Written by the indexing model from the issue text.