[Feature request] Have a way to handle subpaths in archives

[kit-ty-kate]

When users have their OCaml binding or project in a sub directory in their source repository there might be a disparity between the opam file they have in the source repository and the opam file they have to submit to opam-repository.
For instance, with the following source tree that mono-repo bindings can adopt:

.
├── ocaml
│   ├── lib
│   │   ├── pkg.install
│   │   ├── pkg.ml
│   │   ├── pkg.mli
│   │   ├── pkg.opam
│   │   ├── dune
│   │   ├── dune-project
[...]

The install file is located in a subdirectory and won't be picked up by opam. From opam-repository point of view a copy of the install file to the top-level directory is required. However from opam pin point of view there is no easy way of using the same opam file as for instance opam pin add --recursive . would enter the ocaml/lib directory and not stay in the top-level directory for the file copy needed by opam-repository (cp ocaml/lib/pkg.install .) to succeed.

This issue could be addressed in several ways. For instance:

• We could handle every %{name}%.install from any subdirectories
• We could add a subpath field in the url section and get the same behaviour as Recursive & subpath based pin #3499

Both of these have their force and weaknesses.

Issue encountered in ocaml/opam-repository#16361

[dra27]

See also #3880

[dra27]

@rjbou - is this worth looking at as part of the enabling of subpath-pinning in 2.2.0? If not, we can put it back to the Feature Wish project!

[rjbou]

In fact, everything is here (need to review subpath pinning code) to be able to support it. Main target here is opam repository, it needs to allow opam files with subpath field.