When a library is not found, dune suggest to run dune external-lib-deps --missing <targets>. The suggested command will print an approximation of the the list of all external libraries that are missing. This is useful as it avoids having to ping-ping between dune build and opam install to install the missing libraries one by one.

However, we could collect the library names from the various "library not found errors" and print a single hint with the list of libraries. This would still work in stages, but with less stages than when handling errors one by one. Overall, this should provide a similar feature to users but at a smaller maintenance cost for us.