gnu: Rename texlive module to tex.

gnu: Rename texlive module to tex.

* gnu/local.mk (GNU_SYSTEM_MODULES): Rename texlive.scm to tex.scm.
* gnu/packages/texlive.scm: Rename file to tex.scm.
* gnu/packages/tex.scm: Renamed from texlive.scm.
* gnu/packages/algebra.scm: Adjust accordingly.
* gnu/packages/docbook.scm: Likewise.
* gnu/packages/engineering.scm: Likewise.
* gnu/packages/gettext.scm: Likewise.
* gnu/packages/gnome.scm: Likewise.
* gnu/packages/lisp.scm: Likewise.
* gnu/packages/maths.scm: Likewise.
* gnu/packages/music.scm: Likewise.
* gnu/packages/ocaml.scm: Likewise.
* gnu/packages/photo.scm: Likewise.
* gnu/packages/plotutils.scm: Likewise.
* gnu/packages/python.scm: Likewise.
* gnu/packages/scheme.scm: Likewise.