factor 'tcRnModuleTcRnM' out of 'tcRnModule' and export it

'tcRnModuleTcRnM' builds the TcRnM monad computation that is the bulk of
'tcRnModule'

This commit makes it simpler for courageous Core plugins to invoke the
typechecker. In particular, they no longer need to copy-and-paste what is now
the body of 'tcRnModuleTcRnM'. The key change is that the 'tcRnModuleTcRnM'
computation can be run with (a hypothetical) 'initTcFromCoreM' instead of
'initTc'.