pkg: add binary dev tools repo

[gridbugs]

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Adds a repo of binary dev tools to dune so that dev tools can be quickly installed without the need to download and compile their entire dependency cone on supported platforms. Currently ocamlfind and ocamllsp are included in the repo.

[rgrinberg]

This repository is not (yet?) usable outside of dev tools? If that's the case, we shouldn't make it visible in the workspace at all. Ideally, this repository should only be used when installing dev tools.

[gridbugs]

Updated so that the binary repo is only used when solving dev tools.