rule: allow usage of Label struct with Rule APIs

[t-8ch]

What type of PR is this?

Feature

What package or component does this PR mostly affect?

rule

What does this PR do? Why is it needed?

The Label is a convenient structure for custom logic. Allow its use with the Rule APIs so the user can directly feed structured data back into Gazelle.

[t-8ch]

An alternative would be to introduce an interface to make the conversion pluggable.

[fmeum]

An alternative would be to introduce an interface to make the conversion pluggable.

We could look into that if more types come up. This particular one seems clear enough since Label(...) isn't available in BUILD files.

[t-8ch]

An alternative would be to introduce an interface to make the conversion pluggable.

Turns out there is already

// BzlExprValue is implemented by types that have custom translations
// to Starlark values.
type BzlExprValue interface {
	BzlExpr() bzl.Expr
}

[fmeum]

Do you want to submit a follow-up PR that implements this interface for Label instead?

[t-8ch]

The comments was mostly meant as documentation for later visitors.
But yeah, that would make sense. I'll submit one.
Although I'm not sure if the dependency of "label" to "value" is better than the other way around.