backtrack - drop the parser combinators

I thought I could get away with a few simple parser combinators because
I don't need to recover the full AST of the user prelude, I only need to
divide the user prelude into four sections. But it turns out there are
many details such as nested comments and multi-line import lists which
would require more subtle parser combinators. So I will try a different
approach based on haskell-src-exts's SrcLoc annotations.
move HaskellModule to HaskellModule.Base

to make room for HaskellModule.Parse
extract info via haskell-src-exts

the part which divides the HaskellSource into pieces is hardcoded right
now, but the list of modules and extensions should work.