Make the revdeps counter more accurate of actual (recent) use

[kit-ty-kate]

This tries to make the revdeps counter a bit more accurate. For example, currently camlp4 is still at the top of the page with 2111 revdeps. However this is not true for recent versions of the compiler (only 70 in this example).

This proposal makes it so that the counter is now valid for recent uses.

There are a few caviats to that:

• this is slower (2 to 3 times)
• this is even slower on opam 2.1
• this is less accurate if what the user care about is older version of the compiler