packages: Set dependency origin in generate-ucl.lua

Commit 822fd5606748 changed the origin of packages from "base"
to "base/<package name>", but the old origin was still used for
dependencies.

Aside from being wrong, this confuses pkg <= 2.3.1 and causes
"pkg info -d" to show the dependencies as "<pkgname>-(null)"
instead of the correct version string.

Remove the explicit origin from dependencies in the UCL files,
and instead populate this during the build in generate-ucl.lua.
In the (very unlikely) case that an explicit origin is still
needed, don't overwrite one if it's already present.

MFC after: 1 day
Fixes: 822fd5606748 ("pkgbase: set origin for each packages as base/FreeBSD-*")
Reported by: bapt
Reviewed by: cperciva, bapt
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52892