hookedPreProcessors do not re-run preprocessors when their code is changed

I've added a preprocessor to the build hooks in `Setup.hs` like `simpleUserHooks{ hookedPreProcessors = (".myext", myPreprocessor):knownSuffixHandlers }`.

When I change the logic of `myPreprocessorHandler` and run `runhaskell Setup.hs configure && runhaskell Setup.hs build`, the preprocessor does NOT get rerun on all `.myext` files. It only gets rerun when I touch the `.myext` files, or obviously after `runhaskell Setup.hs clean`.

This makes it difficult/confusing to iterate when writing the preprocessor.

I could understand why it wouldn't be rerun on only `build`, but I'd expect at least `configure` to pick up the change, or doesn't this make sense?

Reproduce the behavior using the Setup.hs hookedPreProcessors configuration, then trace how configure and build decide whether .myext files need preprocessing. Verify whether changing myPreprocessorHandler causes all affected files to rerun, and add coverage for the expected invalidation behavior if the relevant test location is found.

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