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ghc-pkg and ghc already both needed this. I figure it is better to deduplicate, especially seeing that changes to one (FreeBSD CPP) didn't make it to the other. Additionally in !1090 I make ghc-pkg look up the settings file, which makes it use the top dir a bit more widely. If that lands, any difference in the way they find the top dir would be more noticable. That change also means sharing more code between ghc and ghc-package (namely the settings file parsing code), so I'd think it better to get off the slipperly slope of duplicating code now.
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