Provide and use 'tex' virtual package

[ahesford]

The texlive virtual package has been renamed tex to be more generic and avoid confusion with the new "native" texlive package, and texlive has been named the default provider for the new virtual.

Every texlive*-bin package provides tex with a version based on its own (which is the release year); the new texlive strips the month and day from its year to similarly provide tex with a version by year.

A few texlive dependents that are quick to build have been updated to depend on the new virtual, and I've bumped python3-pyx while I was at it.

This is really motivated by xournalpp, which is ready to go with the new virtual, but I'm holding back because I want this to run through CI and I'm afraid xournalpp will time out.

This will also break lyx which currently depends on virtual?texlive, but that's also ready to go with a new version bump. Again, including lyx here might overrun Travis time limits.

Seeking comments from @q66 and @fosslinux since they shepherded the new texlive as well as @leahneukirchen as texlive*-bin maintainer.

[fosslinux]

Don't worry about CI, CI doesn't like TeXLive's distfile, something about FTP I think

Looks fine to me.

[ahesford]

I've CI is going to fail anyway, I'll just pull everything in one pass. I added the xournalpp and lyx updates and am now testing locally.

[fosslinux]

Hm, this isn't bad per se, but I think texlive metapackages should be the providers, and make texlive-most the default provider. To be fair, texlive does pull in texlive-core, so this does yield a working installation, but it will only work for the most basic tex documents. Thoughts?