Change package distribution scripts to reject package updates which contain ._FOO files

[fingolfin]

These files are created by macOS versions of tar (both bsdtar and gnutar). We should reject package updates containing them. Currently, thees packages have deposited versions with such file:

• matgrp by @hulpke
• transgrp by @hulpke (already reported in March and fixed then)
• QPA 1.30 by @sunnyquiver, see QPA 1.30 tarball contains macOS metadata files, but shouldn't gap-packages/qpa#51

See also https://superuser.com/a/260264 for some background information.

[hulpke]

Why reject?

I agree that these files are unneccessary, ugly, that package authors should aim to remove them, that our repacking scripts should remove them, and that it is awkward from the start to not only pack such files, but have their removal being dependent on an undocumented feature. But do they cause actual problems?

What if Apple decides (as they have done in the past) that the undocumented feature of tar should change with an update (and thus such files reappear)?