Support import-dirs under Cabal flags

[pepeiborra]

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Since the introduction of Cabal flags in haskell-language-server.cabal, gen-hie is not able to generate a cradle that covers all the import dirs in that Cabal descriptor. In particular, the folder plugins/default gets missed because its included conditionally.

I think that implicit-hie should consider all possible import-dirs, regardless of the flag default values.