[GHC] #8489: clean up dependency and usages handling in interface files

[GHC]

#8489: clean up dependency and usages handling in interface files
----------------------------------------------+----------------------------
       Reporter:  errge                       |             Owner:  errge
           Type:  task                        |            Status:  new
       Priority:  normal                      |         Milestone:  7.10.1
      Component:  Template Haskell            |           Version:  7.7
       Keywords:                              |  Operating System:
   Architecture:  Unknown/Multiple            |  Unknown/Multiple
     Difficulty:  Moderate (less than a day)  |   Type of failure:
     Blocked By:  1480                        |  None/Unknown
Related Tickets:                              |         Test Case:
                                              |          Blocking:
----------------------------------------------+----------------------------
 While reviewing #1480, it was found that there are corner cases that are
 not easy to handle.

 This task is about fixing TH module reification to reify direct import
 list, not usages.  For this we may need to extend the current `ModIface`.

 This task also includes cleaning up the comments and code in related
 areas, to quote simonpj: "we need more precise commentary on the fields of
 `HscTypes.Dependencies`, `TcRnTypes.ImportAvails`, and `mi_usages` of
 `ModIface`. For example, I wonder whether the `mi_usages` field could be
 part of the `Dependencies`".

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8489>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler