Add Package Builder for dlang projects

[monyarm]

The goal of this PR is to create a package builder similiar to buildNpmPackage, buildRustPackage, buildGoPackage and others.

A loose summary of necessary subtasks would be:

• [x] Keep the source code of Dub, so it can later be added as a search path.
• [x] Create a script (with Nix, dlang or bash), which will download the names and versions of each dub package, generate hashes, and output it to a json file.
• [x] Create a CI job with a cron job to run weekly, and run the above script, creating a PR when it does.
• [x] Create a basic buildDubPackage function, which can build dependencyless projects.
• [x] Add dub.selections.json dependency resolution
• [x] If dub.selections.json doesn't exist in the repo, require it to be manually supplied.
• [x] Allow choice of dub version
• [x] Implement test running
• [x] Create packages for a number of popular dlang projects, to serve as an example of buildDubPackage
• [x] Write proper documentation for buildDubPackage
• [x] Ask @PetarKirov for a code review, and additional suggestions

See #64 for old version

[dukc]

Impressive work to do all this in such a short time, especially if you really did this all on your own time!

However, I have an issue. I don't think we should include every DUB package as a nix derivation here. It means that anyone who clones this repo has to download all 2K+ of them.

A script to generate them oneself is fine, if we want to go further we can make another repo to cache the generated DUB package derivations.

ping @PetarKirov @monyarm .

[github-actions[bot]]

Thanks for your Pull Request!

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[dukc]

ping @monyarm All items in the list are checked. Does it mean it's ready for review now?

ping @PetarKirov Your comments also wanted. Or have you already reviewed this offline?