Amend TcPluginM interface

Summary:
Expose some new functions in TcPluginM and remove one, in the light of
experience writing plugins.  In particular, I've removed lookupRdrName
because using it to import modules containing instances leads to subtle
bugs; I've expanded on the lookupRdrNameInModuleForPlugins comments.

Test Plan: validate

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: goldfire, ezyang, carter, thomie, yav, gridaphobe

Differential Revision: https://phabricator.haskell.org/D553

Conflicts:
	compiler/typecheck/TcSMonad.hs