feat: add opam source

[dpeukert]

This PR adds a new opam source for retrieving package versions from opam repositories, used in the OCaml ecosystem.

Example config and output:

[omigrate]
source = "opam"
pkg = "omigrate"

[I 12-15 00:44:44.786 core:416] omigrate: updated to 0.3.2 url=https://opam.ocaml.org/packages/omigrate/omigrate.0.3.2

Using other repositories than the official one is also supported (but without URLs, as there is no universal URL structure):

[coq-abp]
source = "opam"
pkg = "coq-abp"
repo = "https://coq.inria.fr/opam/released"

[I 12-15 00:46:26.971 core:416] coq-abp: updated to 8.10.0

[dpeukert]

The test failure is unrelated.