Add `-fignore-export-list` flag for use in GHCi

Currently GHCi uses the mi_globals field of ModIface to provide the following features:

1. Non exported names are visible and can be used from the repl
2. The import environment of loaded modules is replicated in the repl, so that if you import M in a file and then load the file in GHCi, symbols from M will be available in the repl

Unfortunately using mi_globals for this can leak a lot of memory: #22832 (closed)

Instead I propose to introduce a new flag -fignore-export-list that makes GHC behave as if the export list isn't present while compiling a module. This can be used by GHCi to provide feature 1.

For feature 2 I propose that we have special logic in GHCi to contruct an RdrEnv from the header of the currently loaded module(s).

This way, we can avoid keeping mi_globals around for all the modules in a big GHCi session with a lot of files, and instead only construct the RdrEnv for a module that is :loaded into the repl, from its header, avoiding keeping the RdrEnv around for its transitive dependencies.