coverage use [run] source_pkgs= instead of source= (Chia-Network#14164)

This avoids some collisions in some coverage reports between files in `chia/` and `tests/`.  Minimal example in https://github.com/altendky/example001 and "real" trouble observed in https://github.com/Chia-Network/chia-blockchain/pull/14141/files.

.coveragerc

@@ -1,7 +1,7 @@
 [run]
 branch=True
 relative_files=True
-source=
+source_pkgs=
   chia
   tests
 concurrency=multiprocessing