Nice, this ties directly into one of the thornier problems in the PEP 432 bootstrapping work, which needs to set up the core import system in Py_BeginInitialization, but delay setting up the rest of it until Py_EndInitialization.

Your patch achieves this by moving everything that belongs in the latter part of the operation to the "import _frozen_path_importer" step.

As far as naming goes, I'd suggest referring to the two import subsystems as "internal imports" and "external imports". The key aspect of builtin and frozen modules is that they're an inherent part of the interpreter itself - if you have an interpreter, you have all of its buitin and frozen modules natively available. By contrast, setting up the external import machinery requires that the interpreter first be configured to appropriately communicate with the host system.