Drop redundant flags from stack.yaml

[adamgundry]

Since #5 we have supported specifying a resolver to avoid redundant packages, but currently we redundantly specify the flags for packages present in the snapshot. I don't think it should be very hard to drop redundant flags though, and it would make the generated stack.yaml easier to work with.

[adamgundry]

The resolver YAML files from https://github.com/commercialhaskell/stackage-snapshots/ describing the Stackage snapshots include some flag values, but seemingly not all of them. I'm guessing these might be flags that were specified manually by the Stackage curators?

Thus we have the problem that cabal2stack cannot easily know the flag values used by the resolver, and hence it doesn't know which values may need to be part of the stack.yaml file.