dune-release opam submit uses synopsis of a random opam package

[hannesm]

When I dune-release opam submit in the ocaml-tls (https://github.com/mirleft/ocaml-tls) repository, the generated PR contains the synopsis of the tls-mirage opam package, though I'd prefer it to contain the contents from the tls opam package. see ocaml/opam-repository#16984

I think there's a general issue with multiple opam packages in a single repository (see #244). I'd suggest to denote a "main" opam package (and as I understand, this is the one without -: irmin in the https://github.com/mirage/irmin repository, dns in https://github.com/mirage/ocaml-dns, dune in https://github.com/ocaml/dune, tls in https://github.com/mirleft/ocaml-tls) which content (synopsis, homepage, etc.) is used where needed.

I know #249 has been merged which likely modifies the behaviour slightly (though not clear to me whether this is good now). I used dune-release 1.4.0 here.

[hannesm]

..or use the data in dune-project..

[gpetiot]

Using the data in dune-project can be a stable solution, I will have a look.