Create a real module kiwixbuild and make it installable. Split files.

Make kiwix-build installable using pip.
Module is now called `kiwixbuild` because `kiwix-build` is not a valid
python identifier.

Also split toolchains in separated directory.

kiwixbuild/__main__.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python3
+
+from . import main
+
+
+if __name__ == "__main__":
+    main()