Add (source ...) stanzas to subtree dune-projects
38 subtrees lacked a (source ...) stanza in their dune-project, so
their generated .opam files had no dev-repo: field. That broke
downstream tooling that uses dev-repo to identify the subtree
(notably the root.opam external-dep filter, which couldn't tell
internal packages from external ones).
Add (source (tangled gazagnaire.org/<subtree>)) to each, matching
the URL configured in sources.toml. dune regenerates dev-repo:,
homepage: and bug-reports: in every affected .opam.