[GHC] #15826: Allow registering (Source)Plugins through the GHC API

[GHC]

#15826: Allow registering (Source)Plugins through the GHC API
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        Reporter:  DanielG           |                Owner:  DanielG
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHC API           |              Version:  8.7
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  D5278
       Wiki Page:                    |
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Comment (by mpickering):

 The reason for the existence of the `ModIface` is for recompilation
 checking. How do you imagine recompilation checking working for functions
 added using the API?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15826#comment:4>
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