Negated GHC flags don't cancel out in package hash

As a concrete example, I enable `ghc-options: -haddock` in my global config, and pass `--ghc-options=-no-haddock` when cross-compiling. As a result, much of my store has `cabal-hash.txt`s with `ghc-options: -haddock -no-haddock`, which is the same in principle as passing no GHC flags at all.

This has the potential to cause some cache misses.

Start by tracing where cabal-hash.txt records ghc-options and how global configuration is combined with --ghc-options. Reproduce the -haddock/-no-haddock example, then verify that the resulting package hash matches the equivalent build with no GHC flags and cover the behavior with focused regression testing.

Written by the indexing model from the issue text.