Avoid generating redundant constraints in new-freeze
Avoid generating redundant constraints in new-freeze

We sometimes have to union constraints from different places, but we
should simplify those before rendering so we don't add needless
redundancy like "aeson-pretty ==0.8.1 || ==0.8.1". So we just use
simplifyVersionRange after unioning things.

Fixes #4002

mydaum/cabal,mydaum/cabal,themoritz/cabal,themoritz/cabal,themoritz/cabal