This does not special optimisation to minimise the data.  I guess this
is also a good point to hear objections to the current way the VC data
is handled.  Here is an example entry from "archive-contents", note the
new `:upstream' entry in the metadata field (the last element of the
vector):

--8<---------------cut here---------------start------------->8---
 (setup .
        [(1 3 0)
         ((emacs
           (26 1)))
         "Helpful Configuration Macro" tar
         ((:url . "https://git.sr.ht/~pkal/setup")
          (:keywords "lisp" "local")
          (:maintainer "Philip Kaludercic" . "~pkal/public-inbox@lists.sr.ht")
          (:authors
-->        ("Philip Kaludercic" . "philipk@posteo.net"))
          (:upstream Git "https://git.sr.ht/~pkal/setup" nil nil)
          (:commit . "eece09d1151fd641f31d738b8c62742918993e95"))])
--8<---------------cut here---------------end--------------->8---

or

--8<---------------cut here---------------start------------->8---
 (smalltalk-mode .
                 [(4 0)
                  nil "Major mode for the GNU Smalltalk programming language" tar
                  ((:maintainer "Derek Zhou" . "derek@3qin.us")
                   (:url . "https://elpa.gnu.org/packages/smalltalk-mode.html")
-->                (:upstream Git "https://git.savannah.gnu.org/git/emacs/elpa.git" nil "externals/smalltalk-mode")
                   (:commit . "570b705db9a02bb48cd61652639401715f419447"))])
--8<---------------cut here---------------end--------------->8---

The value corresponding to :upstream is a list of the form

    (VC-BACKEND REPOSITORY-URL SUBDIRECTORY BRANCH)

I have been wondering if it would be better to use a {a,p}list instead,
which would be more flexible if changes would become necessary in the
future.  Perhaps I am even missing something right now?

At the same time I don't mean to burden the ELPA server by increasing
the size of the file that is regularly queried.  Perhaps REPOSITORY-URL
could have special values such as `:url' to indicate that the package
URL is the same as the repository and `:elpa'/`:nongnu' to indicate that
the default repository is being used.  Or is this just premature
optimisation?