scala:annotate imports puts comments in a strange/unexpected place.

[pcj]

A user complained about this.

Rather than annotating the RHS of the srcs attribute, annotate the rule (or something else).

Depends on bazel-contrib/bazel-gazelle#1390.

[pcj]

Snippet for existing_scala_rule.go:

assign := r.AttrAssignExpr("srcs")
if assign != nil {
	annotateImports(imports, &assign.Comments, sc.shouldAnnotateImports(), sc.shouldAnnotateUnresolvedDeps())
}

[pcj]

Consider adding a marker line in case comments already exist:

	comments.Before = append(comments.Before, build.Comment{Token: "# ≡[scala imports]≡"})