Switch most of ImportRefResolver to non-member functions (#4584)

In preparation for further refactoring, switch away from member
functions for most of `ImportRefResolver`.

Split `ImportRefResolver` into a context class that exposes the value
stores for the source and destination files, and a derived class that
maintains a worklist. The idea is to statically enforce that functions
that take `ImportContext` cannot accidentally add new work, because they
don't have access to the work queue.