Fixes to dealing with the export of main

It's surprisingly tricky to deal with 'main' (#19397). This
patch does quite bit of refactoring do to it right. Well,
more-right anyway!

The moving parts are documented in GHC.Tc.Module
   Note [Dealing with main]

Some other oddments:

* Rename tcRnExports to rnExports; no typechecking here!

* rnExports now uses checkNoErrs rather than failIfErrsM;
  the former fails only if rnExports itself finds errors

* Small improvements to tcTyThingCategory, which ultimately
  weren't important to the patch, but I've retained as
  a minor improvement.