Add and execute script to clean stale files from build area

For incremental builds, deleting python modules (or entire packages) can lead to stale .pyc files in the binary directory, which will be used by python despite the original source having been deleted. Make sure during the cmake configuration that we do not have any such stale files in the binary directory.

@akraszna this is an initial attempt to solve the problem we discussed over email. Also adding @smh and @rbielski for information and some additional eyes on this code.