install without opam

[treinen]

Hello, a wishlist request: could you provide an install target (maybe with a different name if necessary) which does not rely on opam? Installing opam on a package building daemon just for the sake of installing ppx-derivers seems an overkill (and would require network access during the build process). -Ralf.

Note that it's only the opam-installer tool that's needed, not all of opam.

@bobot has been looking at having jbuilder install the files directly without using an external tool.