------------------------
    Reorganisation of hi-boot files
  	------------------------

The main point of this commit is to arrange that in the Compilation
Manager's dependendency graph, hi-boot files are proper nodes. This
is important to make sure that we compile everything in the right
order.  It's a step towards hs-boot files.

* The fundamental change is that CompManager.ModSummary has a new
  field, ms_boot :: IsBootInterface

  I also tided up CompManager a bit.  No change to the Basic Plan.

  ModSummary is now exported abstractly from CompManager (was concrete)

* Hi-boot files now have import declarations.  The idea is they are
  compulsory, so that the dependency analyser can find them

* I changed an invariant: the Compilation Manager used to ensure that
  hscMain was given a HomePackageTable only for the modules 'below' the
  one being compiled.  This was really only important for instances and
  rules, and it was a bit inconvenient.  So I moved the filter to the
  compiler itself: see HscTypes.hptInstances and hptRules.

* Module Packages.hs now defines
    data PackageIdH
    = HomePackage 		-- The "home" package is the package
 				-- curently being compiled
    | ExtPackage PackageId	-- An "external" package is any other package

   It was just a Maybe type before, so this makes it a bit clearer.

* I tried to add a bit better location info to the IfM monad, so that
  errors in interfaces come with a slightly more helpful error message.
  See the if_loc field in TcRnTypes --- and follow-on consequences

* Changed Either to Maybes.MaybeErr in a couple of places (more perspicuous)