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.