[2.1.0~alpha] opam admin cache doesn't show the full url for some packages

[kit-ty-kate]

Using opam admin cache with opam master (3def1a6 with a few extra fixes) shows quite a lot of urls being replaced by (https://). For instance:

$ opam admin cache ~/.cache/opam
[...]
[graphics.4.06.1] synchronised (https://)
[pa_test.109.45.00] synchronised (https://ocaml.janestreet.com/ocaml-core/109.45.00/individual/pa_test-109.45.00.tar.gz)
[lablgtk3.0.beta2] synchronised (https://download.ocamlcore.org/lablgtk/Lablgtk3/3.0.beta2/lablgtk-3.0.beta2.tar.gz)
[alcotest.1.1.0] synchronised (https://)
[osx-acl.0.1.0] synchronised (https://github.com/dsheets/ocaml-osx-acl/archive/0.1.0.tar.gz)
[ppxx.1.1.0] synchronised (https://bitbucket.org/camlspotter/ppxx/get/1.1.0.tar.gz)
[jupyter-archimedes.2.3.2] synchronised (https://)
[...]

If this means that the archive has already been downloaded previously, could we get a more explicit log message?

[kit-ty-kate]

When calling opam admin cache on an already populated cache, the synchronised (https://) appear everywhere. This was not the case in previous stable versions of opam.

[rjbou]

This message is wrongly not clear, as it is here to notify that the archive is already cached, but it makes me wonder if it should print a message when it is already cached.