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

[GHC]

#13753: Improve GHC's ghc package environment lookup logic
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        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):
       Wiki Page:                    |
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Comment (by gershomb):

 Along with this, GHC and GHCI should always notify when loading with a
 package-env file. The default lookup paths make it too easy to
 accidentally run with a file when you don't realize you are, causing
 potentially strange behavior. Now that cabal new-* commands generate these
 files by default, I imagine there will be many people confused by this.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13753#comment:4>
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