Allow building only dependencies of a target

Desired Behavior

Colleagues of mine have often asked me if there is a way to tell dune to build only the dependencies of some target. This has come up again today in the context of a Coq project, where we have relied on the following hack.

To build all the dependencies of a Coq file (which is useful to then load it into the Coq toplevel), on can use dune coq top --toplevel=true -- dir/file.v. This has the same effect as building all the dependencies for file dir/file.vo, without building dir/file.vo itself.

However, this approach is limited to cases where you want to build the deps for a single file. It would not be reasonable to allow several file parameters to dune coq top, since that does not really make any sense from the point of view for the toplevel.

A better option would be to add a --deps-only argument to dune build, so that one can run, for example, dune build --deps-only dir/file1.vo dir1/file2.vo to build all the dependencies of dir/file1.vo and dir/file2.vo.

Start at the dune build command and compare its target handling with dune coq top --toplevel=true. Trace how multiple targets and their dependency closure are processed. Done means a --deps-only option builds dependencies for targets such as dir/file1.vo and dir1/file2.vo without building those targets themselves.