gnu: Move most packages from guile.scm to new module.

* gnu/packages/guile.scm (artanis, guildhall, guile-aspell, guile-bash,
guile-8sync, guile-daemon, guile-dsv, guile-fibers, guile-syntax-highlight,
guile-sjson, guile-colorized, guile-pfds, guile-aa-tree, guile-simple-zmq,
jupyter-guile-kernel, guile-sparql, guile-debbugs, guile-email,
guile-debbugs-next, guile-newt, guile-mastodon, guile-parted, guile-xosd,
guile-dbi, guile-dbd-sqlite3, guile-config, guile-hall, guile-ics, guile-wisp,
guile-sly, g-wrap, guile-miniadapton, guile-reader, guile2.2-reader,
guile-ncurses, guile-ncurses/gpm, guile-lib, guile2.0-lib, guile2.2-lib,
guile-minikanren, guile2.0-minikanren, guile2.2-minikanren, guile-irregex,
guile2.0-irregex, guile2.2-irregex, haunt, guile2.0-haunt, guile2.2-haunt,
guile-redis, guile2.0-redis, guile2.2-redis, guile-commonmark,
guile2.0-commonmark, guile2.2-commonmark, mcron, mcron2): Move these variables
from here...
* gnu/packages/guile-xyz.scm: ...to this new file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* gnu/installer.scm,
gnu/packages/bioinformatics.scm,
gnu/packages/ci.scm,
gnu/packages/gtk.scm,
gnu/packages/guile.scm,
gnu/packages/mail.scm,
gnu/packages/package-management.scm,
gnu/packages/skribilo.scm,
gnu/packages/web.scm,
gnu/services/mcron.scm: Update module references.