Promote (public_name <package>.<subpackage>) as an alternative to creating a new <package>.opam file

[kit-ty-kate]

opam-repository regularly receives PRs where a number of packages have the exact name dependencies (modulo dependencies between them) and I feel like in most cases adding more packages to the opam namespace is not required (e.g. the tezos- and octez- packages).

Currently, dune's error messages only invite people to create <package>.opam files and never mention the possibility of subpackages anywhere. For example:

• https://github.com/ocaml/dune/blob/a62608f443eed2f71518c03194e7f9612f17c419/src/dune_rules/stanza_common.ml#L32
• https://github.com/ocaml/dune/blob/a62608f443eed2f71518c03194e7f9612f17c419/src/dune_rules/stanza_common.ml#L63

I feel like mentioning subpackages first in those error messages would be really helpful for users (less generated opam files with duplicated informations and packages to deal with when publishing) and opam-repository maintainers (less packages to maintain and review and CI would be less broken all the time)

[rgrinberg]

That sounds reasonable.