Use tcRnImports rather than rnImports with GHCi "import" statement: fixes #4832

The bug here was that just using rnImports does not ensure that any dependent
orphan modules are loaded, so instances declared by such modules will not be
usable from the GHCi command line after an "import".

This did not affect the :m syntax because it takes a different code path and
uses getModuleExports directly, which contains its own calls to the orphan-module
loading stuff.