Fix for method tasked with __py_build__/defs integrity checks

[jim-hart]

Fixed error with FileExpander.verify_defs_integrity()

While FileExpander.verify_defs_integrity() was correctly detecting changes to the main defs directory under src/, when trying to update files under __py_build__/defs/ it would fail due to use of shutil.copytree(). Issue was resolved by looking for individual file differences and copying via shutil.copy2() on an as-needed basis. New method of detecting changes is more reliable and non-destructive.

Also condensed some overly verbose doc-strings as everything that was deleted is already covered in the module overview.

[daboross]

Thanks!