Build python3-snowballstemmer from this source

python3-snowballstemmer package is currently built from the snowballstemmer tarball from PyPI. However that tarball is generated from the main snowball repository, using the dist_libstemmer_python make target. So instead of having a separate source package, it is better to build python3-snowballstemmer directly from this source. This way, if we backport some fix for snowball (or add a new language), it will automatically apply to the python3-snowballstemmer package too.

Cc @josue — when this is merged/uploaded, I will request removal of python-snowballstemmer source from Debian.

Also this MR removes use of dh-exec which is no longer needed with debhelper 13.