Move withTcPlugins to TcRnDriver, removing need for hs-boot file

This makes the caller of initTc responsible for loading plugins if
required, rather than passing a boolean flag indicating whether to
load them.  Hence withTcPlugins can move to TcRnDriver, avoiding
the need for DynamicLoading.hs-boot.
Move core2core plugins to a new module Plugins
Make TcPlugin part of Plugin so we can eliminate -ftc-plugin