[GHC] #13753: Improve GHC's ghc package environment lookup logic

[GHC]

#13753: Improve GHC's ghc package environment lookup logic
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4690
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by hvr):

 * differential:   => Phab:D4690


Comment:

 Phab:D4690 implements the first half of this feature request, i.e. the
 ability to opt out from the package environment feature.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13753#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler