[GHC] #12637: ghc-pkg: Allow unregistering multiple packages in one call

[GHC]

#12637: ghc-pkg: Allow unregistering multiple packages in one call
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           Reporter:  nh2            |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This can save lots of time when the cost to invoke a single GHC command is
 high, for example through docker.

 An example user of this would be `stack build`, which will unregister all
 relevant packages if e.g. `--extra-lib-dirs` has changed. Currently it has
 to do that through multiple `ghc-pkg unregister` operations, which can
 take about 2 seconds per call when done through a docker image. If we
 could unregister multiple packages at once, such use cases would have to
 pay the 2 seconds penalty only once.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12637>
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