ocamlfind, dune, opam, oh my

Just a short note to record that there are various differences between e.g. dune install, and opam install. Indeed, with the current p0 repo, dune install seems to install the library (which can be seen by ocamlfind and dune installed-libraries commands), but not by opam (which doesn't list any p0_lib package).

It is not quite clear to me what the semantics of "dune install" is, and why it differs from "opam install". I get that dune, opam and ocamlfind are all different tools, but it seems strange that "dune install" will install files into the opam switch, but won't install the actual opam package. Another one of life's mysteries I guess.

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