As discussed in the last iteration, this would be bad for performance. At the very least, I'd only want to support this in XPIProvider if it was only possible when in a special developer mode. But IMO, just supporting this in the module loader itself (again, only as a developer option) would be easier.

If its a developer-only feature, we don't need to worry nearly as much about performance, and we won't need to persist anything (ie, save a copy of a generated modules.json at runtime).