WIP: broadband-modem-qmi: make location providers pluggable

This prepares for an upcoming alternative implementation using the LOC service. The code in mm-broadband-modem-qmi-location-pds.c is nearly copied verbatim from src/mm-broadband-modem-qmi.c, only the peek and ensure qmi client functions have been renamed.

This MR has not been tested, as I do not have access to the relevant hardware. A prototype for the LOC service based on these changes seems to work so far though.