ghci: only keep the GlobalRdrEnv in ModInfo

The datatype GHC.UI.Info.ModInfo used to store a ModuleInfo, which includes a TypeEnv. This can easily cause space leaks as we have no way of forcing everything in a type environment.

In GHC, we only use the GlobalRdrEnv, which we can force completely. So we only store that instead of a fully-fledged ModuleInfo.

I haven't opened a ticket for this issue yet, but storing a full ModuleInfo here seems like a liability. However, I'm hesitant to remove it outright in case users were relying on the presence of the type environment. Edit: it was pointed out to me that this code is purely internal to GHCi, so there is no risk of anyone depending on having access to information I am removing.