Make GHC store language extensions in `.hi` files

[martijnbastiaan]

As @mmzx noted in #722 (comment), we can't warn users about NoMonoLocalBinds if the code we're fetching comes from external packages (.hi files) simply because GHC doesn't store what language extensions modules were built with. We should investigate whether we can extend GHC to support this.

[christiaanb]

GHC 8.8 has support for .hie files now, which is probably where you want to store them in parsed form. For 8.8, we can get them from the unparsed source: https://downloads.haskell.org/~ghc/8.8.1/docs/html/libraries/ghc-8.8.1/HieTypes.html#t:HieFile; for 8.10 it would be nice to get them in the parsed form: you'll have to open a ticket for that: https://gitlab.haskell.org/ghc/ghc/issues