rule: allow usage of Label struct with Rule APIs (bazel-contrib#1705)

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.

rule/BUILD.bazel

@@ -32,6 +32,7 @@ go_test(
     ],
     embed = [":rule"],
     deps = [
+        "//label",
         "@com_github_bazelbuild_buildtools//build",
         "@com_github_google_go_cmp//cmp",
     ],

rule/value.go

@@ -22,6 +22,7 @@ import (
 	"sort"
 
 	bzl "github.com/bazelbuild/buildtools/build"
+	"github.com/bazelbuild/bazel-gazelle/label"
 )
 
 // KeyValue represents a key-value pair. This gets converted into a
@@ -125,6 +126,7 @@ func (s SelectStringListValue) BzlExpr() bzl.Expr {
 // a Bazel build file. The following types of values can be converted:
 //
 //   * bools, integers, floats, strings.
+//   * labels (converted to strings).
 //   * slices, arrays (converted to lists).
 //   * maps (converted to select expressions; keys must be rules in
 //     @io_bazel_rules_go//go/platform).
@@ -198,6 +200,8 @@ func ExprFromValue(val interface{}) bzl.Expr {
 				X:    &bzl.LiteralExpr{Token: "glob"},
 				List: globArgs,
 			}
+		case label.Label:
+			return &bzl.StringExpr{Value: val.String()}
 		}
 	}
 

rule/value_test.go

@@ -19,6 +19,7 @@ import (
 	"testing"
 
 	bzl "github.com/bazelbuild/buildtools/build"
+	"github.com/bazelbuild/bazel-gazelle/label"
 	"github.com/google/go-cmp/cmp"
 )
 
@@ -90,6 +91,10 @@ func TestExprFromValue(t *testing.T) {
 				},
 			},
 		},
+		"labels": {
+			val:  label.New("repo", "pkg", "name"),
+			want: &bzl.StringExpr{Value: "@repo//pkg:name"},
+		},
 	} {
 		t.Run(name, func(t *testing.T) {
 			got := ExprFromValue(tt.val)