[GHC] #3268: implement the Cabal ${pkgroot} spec extension

[GHC]

#3268: implement the Cabal ${pkgroot} spec extension
---------------------------------+------------------------------------------
    Reporter:  duncan            |        Owner:              
        Type:  feature request   |       Status:  new         
    Priority:  normal            |    Milestone:  7.2.1       
   Component:  Package system    |      Version:  6.10.2      
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:  Unknown     
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------

Comment(by duncan):

 Ok, so we'll make both `dump` and `describe` use the unexpanded form by
 default and provide `--expand-pkgroot` which should generate the previous
 format exactly.

 `ghc-pkg field` will by default expand the `${pkgroot}` but that can be
 altered via `--no-expand-pkgroot`.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3268#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler