RFC: changes to -i flag for finding source files

[Simon Marlow]

I want to propose a simple change to the -i flag for finding source 
files.  The problem we often have is that when you're writing code for a 
library that lives deep in the module hierarchy, you end up needing a 
deep directory structure, e.g.

  src/
    Graphics/
      UI/
       Gtk/
         Button.hs
         Label.hs
         ...

where the top few layers are all empty.  There have been proposals of 
elaborate solutions for this in the past (module grafting etc.), but I 
want to propose something really simple that would avoid this problem 
with minimal additional complexity:

   ghc -iGraphics.UI.Gtk=src

the meaning of this flag is that when searching for modules, ghc will 
look for the module Graphics.UI.Gtk.Button in src/Button.hs, rather than 
src/Graphics/UI/Gtk/Button.hs.  The source file itself is unchanged: it 
still begins with "module Graphics.UI.Gtk.Button ...".

The implementation is only a few lines in the Finder (and probably 
rather more in the manual and testsuite), but I wanted to get a sense of 
whether people thought this would be a good idea, or if there's a better 
alternative before I push it.

Pros:

   - simple implementation (but Cabal needs mods, see below)
   - solves the deep directory problem

Cons:

   - It makes the rules about finding files a bit more complicated.
     People need to find source files too, not just compilers.
   - packages that want to be compatible with older compilers can't
     use it yet.
   - you can't use '=' in a source directory name (but we could pick
     a different syntax if necessary)
   - It won't work for Cabal packages until Cabal is modified to
     support it (PreProcess and SrcDist and perhaps Haddock are the only
     places affected, I think)
   - Hackage will need to reject packages that use this feature without
     also specifying ghc >= 7.10 and some cabal-version too.
   - Are there other tools/libraries that will need changes? Leksah?

Cheers,
Simon