Fix backpack typechecking with -j

Motivation

Currently, backpack typechecking is not supported with -j as seen here.

This rather drastically increases compile times for large projects using backpack. It has not been revisited since the original backpack merge.

Proposal

Enable backpack typechecking with -j, addressing whatever attendant issues caused this initial restriction. @ezyang expressed willingness to help out at least at a mentor level.