scripts/pythondistdeps: Do anything only when called as a main script

Note that the code is completely unchanged except for the indentation
under the new if __name__ == "__main__":

Note that this change is necessary, but not sufficient to use the
RpmVersion class.
The init of the RpmVersion class will fail when called from an outside
script, because the `parse_version()` function is lazily imported from
the code outside the class.  However, adding the import of
parse_version() to RpmVersion class is not done right now, because while
we would import it from `pkg_resources`, other scripts might want to
rely instead of the lightweight `packaging` module for the import. Thus
I'm leaving this conondrum to be addressed in the future.

pythondistdeps.py

@@ -143,6 +143,10 @@ def normalize_name(name):
     return re.sub(r'[-_.]+', '-', name).lower()
 
 
+if __name__ == "__main__":
+    """To allow this script to be importable (and its classes/functions
+       reused), actions are performed only when run as a main script."""
+
     parser = argparse.ArgumentParser(prog=argv[0])
     group = parser.add_mutually_exclusive_group(required=True)
     group.add_argument('-P', '--provides', action='store_true', help='Print Provides')
@@ -166,7 +170,6 @@ parser.add_argument('-l', '--legacy', action='store_true', help='Print legacy py
     parser.add_argument('files', nargs=argparse.REMAINDER)
     args = parser.parse_args()
 
-
     py_abi = args.requires
     py_deps = {}