Instead of instructing VUT authors to clean the generated files, make
sure they are automatically picked up. It is safe to upgrade while
still having the CLEAN_FILES rule in Makefile.am, despite "removing"
functionality (removing burden in reality).

Other improvements include a single Python detection even when building
several modules (but the underlying check is cached anyway) and a new
dependency to "nodist" sources for VMOD authors that follow the
recommendations. The "hardcoded" dependency is still present to avoid
breaking existing builds.