Move music transfer main code into main() function

This is both good practice, and also useful for some soon-to-be-added
test cases for the script.

src/.bin/music-transfer.py

@@ -32,6 +32,7 @@ KNOWN_EXTENSIONS: Set[str] = set([
 def munge_path(path: str) -> str:
 	return re.sub(r'[\:*?"|<>]+', '-', path)
 
+def main():
 	if len(sys.argv) != 3:
 		print(f'Usage: {os.path.basename(__file__)} PATHS-FILE DESTINATION-DIR', file=sys.stderr)
 		sys.exit(1)
@@ -156,3 +157,6 @@ for source_file in source_files:
 
 	# TODO: improve this output (number of deleted files, number of new files, correct plural of 'files')
 	print(f'Finished copying {len(source_files)} new files to {destination_dir}')
+
+if __name__ == '__main__':
+	main()