Provide the ability to read dependencies from an opam file

[dysinger]

When building a binary in the style of "examples/opam-dependencies" it would be nice to load an opam file instead of describing all the dependencies (yet again) in a toSpec. Dune (the build tool) does this.

I have a MirageOS unikernel that builds with the command line build tool "mirage." The binary that it spits out may or may not even run on the host that built it (not without a hypervisor anyway.) So I need to load the opam file and build with stdenv.mkDerivation phases.

[timbertson]

I'm a little confused. I understand what you mean, but I don't quite know why you need it. examples/opam-library doesn't make you to specify your deps in nix, it just uses the existing opam file. It sounds like you want to have an opam file but only use it to specify deps, not build instructions?

I also have a mirage project https://github.com/timbertson/passe/, but it's got a bunch of different backends so the build is all in nix, I wasn't able to represent it terribly well in plain opam files. Maybe that's what you're getting at with your second paragraph but I don't entirely follow. If you can't specify your build instructions in an .opam file, why are you using one?