Opam.Fileopam package file access.
Normally opam metadata access is only needed at distribution time and this is handled by Topkg_care.Opam.File using the opam-format library.
However there is one case where we want to be able to access the metadata from Topkg: on pin builds where the watermarking process needs to be run to turn the repo into a pseudo-distribution.
Since we don't want Topkg to have any dependency and that opam currently doesn't allow to consult the fields of arbitrary opam files (see issue #2446) we assume a pin build has the topkg tool installed and call to it to get the opam fields for watermarking (if topkg is unavailable the watermarks are simply undefined).