versioned packages in tlnet

[Karl Berry]

After some discussion and unrelated delays ... 
Norbert and I thought that Brook's idea of "versioning" the TL packages
in tlnet was a good one.
  https://tug.org/pipermail/tex-live/2019-August/044032.html

The idea is that the tlnet/archive/foo.tar.xz files that we have now
become tlnet/archive/foo.rNNNNN.tar.xz, and foo.tar.xz is a symbolic
link to that .rNNNNN file.

Norbert wrote the changes for our infrastructure code,
and we built a test repository yesterday at
ftp://tug.org/texlive/test-tlnet, for anyone who wants to look.
(I strongly advise against actually updating anything but a throwaway
test TL installation with it, although it should work.)

Barring some problem, I plan to make this be the new reality with
tomorrow (Sunday) night's build.

The existing tlmgr programs should not notice any difference, due to the
symlink. We'll update tlmgr anyway soon to try for the versioned file
first anyway, but that's independent.

The only other problem we thought of is if a Windows machine that does
not support symlinks is hosting the repository. That wouldn't work. But
since we have always required symlinks for the
tlnet/update-tlmgr-latest* recovery scripts, it seems there should not
be any instances of this.

We considered trying to do only a few packages first, but that seemed
actually more likely to induce problems, because of special cases in the
code that would then have to be removed. So we're going for it all.

Good luck to us all,
Karl