Track number of installs of packages

[palmskog]

The OCaml OPAM archive tracks the number of installs for each package (version) and displays this information under the header "Statistics", for example for Coq 8.10.2.

To measure popularity of packages and make sure we pay more attention to popular packages, we should try to implement some form of similar tracking of package installs. GitHub already tracks archive downloads, but most Coq OPAM archive installs will hit the cache rather than GitHub.

[Zimmi48]

Note that "statistics" are for "last month" only, so it makes it hard to compare, for instance, downloads of different Coq versions.