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These constructors don't carry any additional semantics.
They were used in `cabal check` implementation, but now already parser
reports errors so we don't need these redundant constructors anymore.

MajorBoundVersion (^>=) has slightly different semantics:
we want be able to relax only `^>= x.y.z` upper bounds.
Therefore it's preserved.

As small change, there's a empty line after multiline fields
in a pretty output.