This is a specific task related to DFDL-623 (edit-compile-test cycle too slow).

Current sbt reruns propgen every time you do a compilation.

This invalidates daffodil-lib, which then must also be recompiled, and that seems to ripple to everything that uses daffodil-lib.

We need to change the sbt scripts so that propgen, which changes only rarely, is rerun only if needed, so as to minimize rebuilding.