Issue #2810: Search upwards for package directory

[Bardur Arantsson]

Hey all,

I've been working a bit on #2810 and would love to have initial review
comments; see

   https://github.com/haskell/cabal/issues/2810
   https://github.com/haskell/cabal/issues/2811 (pull req)

(Btw, would it be possible to assign me to the issue?)

I'd especially like comments on the following:

  - Does this look like a sane approach?

  - Are the any obvious-to-cabal-experts problems here?

  - I'd especially like feedback on the non-trivial FIXMEs.
  (There are some really obvious things I just haven't got
  round to.)

Regards,