Accumulate explicitly which modules to load for 'attachInstances'

The old approach to fixing #469, while correct, consumes a lot of
memory. We ended up with a HUGE 'GblRdrEnv' in 'ic_rn_gbl_env'. However,
'getNameToInstancesIndex' takes that environment and compresses it down
to a much smaller 'ModuleSet'.

Now, we compute that 'ModuleSet' explicitly as we process modules. That
way we can just tell 'getNameToInstancesIndex' what modules to load
(instead of it trying to compute that information from the interactive
context).

(cherry picked from commit 5c7c596c)