If a module layer is created with the defineModulesXXXX methods then the "parents" method that returns the list of parent layers returns a modifiable list, it should return an unmodifiable list. This bug does not impact the boot layer (or the empty layer). Configuration::parents does not have this issue.

As part of this, we should clarify the ModuleLayer.parents() to make it clear that the returned list is unmodifiable. Configuration::parents javadoc is already clear that the returned list is unmodifiable.