Subproject overlap policy is inherited by including projects

Summary

When building something on top of a project which has turned overlap warnings into errors, the project including the subproject will get errors even if that project did not ask for errors.

Steps to reproduce

I discovered this while trying to build a GNOME VM, since the freedesktop-sdk project enables the errors, the gnome-build-meta project is forced to whitelist any overlapping files and can not treat these overlaps as warnings, even though gnome-build-meta did not as for errors here.

Overlaps while building the subproject with errors enabled should be errors, overlaps from the including project which did not enable errors should be warnings.