intial draft NixOS depext support

[RyanGibb]

This addresses #5124 and stems from conversations in #5332.

This is an initial draft to get a conversation started.

Nix's declarative deployment model doesn't match the semantics of the other package managers opam invokes, so it requires some special treatment to be supported.

From conversations on #5332 the most flexible approach is to create a shell.nix and allow the user to decide how to provide the dependencies, such as invoking nix-shell.

This has some limitations, like if the shell is destroyed, and the packages are garbage collected, things will break. For more permanent deployment https://github.com/tweag/opam-nix (or others) can be used to create Nix derivations for opam projects. But this should be sufficient to get depexts on NixOS working for a development environment using an opam switch.

This could possibly also be adapted for Guix.

[RyanGibb]

I'm going to close this in favour of creating a Nix shell like environment which Opam can set the environment variables for: https://github.com/RyanGibb/nix.opam