cli: init with template

[LorenzBischof]

Closes #1718

[liammcdermott]

Thanks for this! I tested this PR using:

devenv init --template github:LorenzBischof/devenv-issue-1718

And it worked as expected.

[LorenzBischof]

If this feature is being considered, we should probably also merge the .gitignore file.

I also thought about maybe forcing a template prefix? We could default to devenv-default and if that does not exist list all the templates that start with devenv-? This would ensure that there can be both flake and devenv templates at the same time.

We could also think about adding a templates option to the devenv.nix file, but that would require committing the generated flake.nix or reimplementing the whole template downloading logic ourselves.

[sandydoo]

@LorenzBischof, this is fine as it is. I don't particularly like coupling ourselves to the flake API, but this feels like a useful feature for some and we can re-implement the behaviuor separately if needed down the line.

Could you please rebase this on main for the builds to pass and I'll merge it.

[domenkozar]

I don't think we should be using flakes for templates, we should add templates top-level attribute to devenv if we want to support it

[LorenzBischof]

@domenkozar Could you give me a quick heads up if the following approach sounds reasonable?

1. Split template reference at #
2. nix flake prefetch with the first part.
3. Generate a Flake from the devenv.nix
4. Use nix flake init --template with the store path and second part after #

This would reuse all the code from Flake templates, but would allow a future refactor if required. It also allows us to validate templates and fail if something is wrong or there are no devenv templates available.

[domenkozar]

We're currently hacking on #1548 at https://oceansprint.org, I think we need that in order to be able to parse flakeref uris and then we can support templates directly from devenv.

It's going to take a while to get this in, but I really don't want to encourage to write flake files for templates and then using devenv.

[LorenzBischof]

The templates can now be specified in the devenv.nix file under the top-level templates option. The basic functionality is implemented, but still needs some cleanup.

Since I have no idea how far along the Snix implementation is, I do not know if it makes sense to continue. Would you consider merging this before the rewrite?