Migrating qserv to the new c++ geometry API required porting a fair amount of code from the python layer and updating the plumbing in the czar. During implementation, the sphgeom was in the process of finding a home, so the sg code was temporarily placed under core/modules.
This ticket covers:
- removing core/modules/sg
- updating code to point at the external sphgeom
- updating build-logic to properly depend on and link with external sphgeom.