Add helper script to create upstream tarballs

Since we cannot use the upstream tarballs (see [1]), add a simple Python script to clone the upstream repository and create a tarball based on the last tag in it. The approach is based on what Guido Günther described in [2].

Following the upstream tag is preferred, as we can package exactly what upstream considered as "release", instead of a snapshot at a random time.

Cloning the upstream repository every time seems wasteful, however:

• its size is only few MB (less than 5 ATM)
• we are sure to use the unmodified sources, with no potentially conflicting tags from an existing local clone

Document this limitation and the new script in README.source.

[1] https://gitlab.com/libosinfo/osinfo-db/-/issues/50
[2] !4 (comment 139324)

Gbp-Dch: Short