Note [Looking up Exact RdrNames]

Exact RdrNames are generated by:

* Template Haskell (See Note [Binders in Template Haskell] in GHC.ThToHs)
* Derived instances (See Note [Auxiliary binders] in GHC.Tc.Deriv.Generate)

For data types and classes have Exact system Names in the binding
positions for constructors, TyCons etc.  For example
    [d| data T = MkT Int |]
when we splice in and convert to HsSyn RdrName, we'll get
    data (Exact (system Name "T")) = (Exact (system Name "MkT")) ...
These System names are generated by GHC.ThToHs.thRdrName

But, constructors and the like need External Names, not System Names!
So we do the following

 * In GHC.Rename.Env.newTopSrcBinder we spot Exact RdrNames that wrap a
   non-External Name, and make an External name for it. This is
   the name that goes in the GlobalRdrEnv

 * When looking up an occurrence of an Exact name, done in
   GHC.Rename.Env.lookupExactOcc, we find the Name with the right unique in the
   GlobalRdrEnv, and use the one from the envt -- it will be an
   External Name in the case of the data type/constructor above.

 * Exact names are also use for purely local binders generated
   by TH, such as    \x_33. x_33
   Both binder and occurrence are Exact RdrNames.  The occurrence
   gets looked up in the LocalRdrEnv by GHC.Rename.Env.lookupOccRn, and
   misses, because lookupLocalRdrEnv always returns Nothing for
   an Exact Name.  Now we fall through to lookupExactOcc, which
   will find the Name is not in the GlobalRdrEnv, so we just use
   the Exact supplied Name.