[GHC] #13128: Refactor AvailInfo to be more sensible

[GHC]

#13128: Refactor AvailInfo to be more sensible
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        Reporter:  mpickering        |                Owner:  mpickering
            Type:  task              |               Status:  new
        Priority:  low               |            Milestone:
       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:  12662
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > In another ticket pacak reported that the size of the build log was over
 100mb because of warnings emitted from ListSetOps

 But are the warnings from usage in `AvailInfo`?  Easy to find out by using
 `HasCallStack` info.   Reason for asking: if the problem is somewhere
 else, it might be a better use of time to fix the somewhere else.

 > The new problem with this ticket now is that backpack signature files
 use the same interface files as regular modules.

 But separating `IfaceAvailInfo` from `AvailInfo` may help this.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13128#comment:5>
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