versioned container testing

[Karl Berry]

Norbert and I have been working on the versioned TeX Live "container"
stuff for a while now (previously requested/discussed at
  https://tug.org/pipermail/tex-live/2019-August/044030.html etc.).
Thus, on the server, tlnet/archive/foo.tar.xz would be a
symlink to foo.r12345.tar.xz instead of the plain file.

A test repository with such symlinks is available at
http://tug.org/texlive/test-tlnet.  I update it pretty much every day
(by hand).

If anyone can give it a try, in whatever way, that would be great. Both
installation and (more importantly) tlmgr updates. I suggest using a
test installation, not anything critical, in case of unexpected problems.

Any up-to-date installation should work with it. An older installation
will need the --self update from current tlnet first, else
there will be mysterious errors from tlmgr. Here is my description of that:
  http://tug.org/texlive/tlmgr.html#verpkg

The test-tlnet repository (i.e., its /archive/ subdir) is "mixed", with
both versioned (recently updated) and unversioned (unchanged) packages.
This seems to work out ok and feels less overwhelming than changing
everything at once. We'll switch all packages to being versioned with
the TL20 release.

The one thing which we know won't work with versioned containers is a
Windows machine being the repository. That is, someone rsync-ing tlnet
to a local Windows machine and then using that local dir as their
server, unless symlinks are supported on the Windows machines involved.
(I am not actually sure if this works now.) I mentioned it in my
followup msg
  https://tug.org/pipermail/tex-live/2019-September/044086.html 
without any objections, so we hope this won't be an issue.

We hope to make this be the new reality in a few more days. So
please try test-tlnet if you can, and thanks. --karl.