texlive-fetch instead of svn-fetch

[Ricardo Wurmus]

Hey Guix,

Tex Live packages often have a complicated source file layout.  They
don’t usually come in a nice self-contained tarball.  Instead they are a
mosaic of files that are sourced from different parts of the Tex Live
SVN tree.  For many Tex Live packages the “source” field is
insufficient, so we add lots of additional origins as native inputs.
This makes for very ugly package definitions.  (See
texlive-fonts-txfonts for an example.)

What do you think about adding a new procedure that can fetch sources
from different locations?  This might be done by extending SVN-FETCH or
by adding a special TEXLIVE-FETCH that fetches sources from different
directories in the source tree, returning them as *one* origin with one
simple hash.

--
Ricardo