openXC7: Import upstream packages, add project

[OPNA2608]

Closes #552

https://github.com/openXC7/toolchain-nix contains upstream's Nix code for the toolchain, but we can't just fetchFromGitHub that & pass everything through as-is

• We can't do import or callPackage any of the downloaded files, because that would re-introduce IFD to the repo after it was previously removed from it treewide.
• We could just copy the src into out and make a shell script that wraps Nix to run nix develop inside of the downloaded checkout, but
  1. nix is dumb and sees nix develop /nix/store/... as "oh, you want me to build a derivation based on this store path… sorry, i can't parse this into a derivation!", necessitating some directory changing shenanigans
  2. It makes it impossible to test that setup in a VM, since that would require internet access for Nix to resolve URLs for downloads… unless we re-introduce IFD and finangle the VM environment to have everything pre-downloaded maybe?

So that left me with one option: Copy the relevant files from the repo into here, and replicate some setup code from the flake to build a wrapper that adds things to the environment, similar to a development shell (whose output we can't just use as-is, because the generated file in the store is commented with "this is an implementation detail, do not have any expectations for this path).

As a result, src is kinda useless, only serving as a pointer to what version of the repo the code was copied from.

Formatting of the copied files differs from upstream due to our pre-commit hook forcing a reformatting.

Really not happy with any of this, but given the situation described above, it is what it is…

[fricklerhandwerk]

@OPNA2608 this is the correct approach from what I gather after reading or participating in discussions with Nixpkgs maintainers. @imincik and @JulienMalka may chip in there with 👍👎.

Nixpkgs (and by extension NGIpkgs) a distribution, and we curate how exactly we integrate the actual code, and that's expressed with the Nix language.

[OPNA2608]

Struggled abit with getting the packages & tests excluded from CI on platforms where some dependencies are unavailable, this is the best that I could figure out.

[fricklerhandwerk]

Makes sense, thank you!