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

[23Skidoo]

@GrayJay writes:

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