Fix init command and support running Mill without existing build.sc

[lefou]

I first added an integration test for the init command to reproduce the issues and make sure the feature now works.

Instead of failing in case there is no build.sc, we now check the targets against a whitelist of tasks (init and version) and continue with an empty setup.

We also accept a missing build.sc in case additional imports are given, which could mean, the imports contain Mill plugins which itself provide commands that could work without a build.sc.

Only when we discover an error afterwards, we add a message about the missing build.sc to the underlying error message. That means, we don't see the spurious "no build.sc" message when everything works as expected.

• Fix Lost ability to install Mill for GHA when build.sc is not at the project root #2660

[lefou]

@lihaoyi Before continuing it would be nice to have your feedback on this issue. Beside initialization of new projects an maybe the version command, I don't see why Mill should run without a top-level build.sc. This has often the potential to leave out directories in unwanted locations. OTOH, we have that init feature, which can't run without this ability.

We could make it an built-in option though. We already have --version to avoid evaluating MainModule.version.

[lihaoyi]

Yeah this goes back to the "how do evaluator commands work" discussion. I'd be fine with either magic whitelisting the init command or baking it in as --init. Since we don't have an overarching principle yet, I expect things would be messy regardless of what we do, and I also expect we'll clean it up later in some new version so I don't have strong opinion on how it's implemented now

[lefou]

I added a whitelist of tasks, so we only continue without a build.sc, if we want to execute a known well-behaving task.