Allow adding arguments to Rules

This allows supporting patterns like `maybe`, which take a
rule-to-instantiate as a positional argument.

rule/rule.go

@@ -880,6 +880,11 @@ func (r *Rule) Args() []bzl.Expr {
 	return r.args
 }
 
+// AddArg adds a positional argument to the rule.
+func (r *Rule) AddArg(value bzl.Expr) {
+	r.args = append(r.args, value)
+}
+
 // Insert marks this statement for insertion at the end of the file. Multiple
 // statements will be inserted in the order Insert is called.
 func (r *Rule) Insert(f *File) {

rule/rule_test.go

@@ -58,7 +58,11 @@ y_library(name = "bar")
 	foo.Delete()
 	bar := f.Rules[1]
 	bar.SetAttr("srcs", []string{"bar.y"})
-	baz := NewRule("z", "baz")
+	loadMaybe := NewLoad("//some:maybe.bzl")
+	loadMaybe.Add("maybe")
+	loadMaybe.Insert(f, 0)
+	baz := NewRule("maybe", "baz")
+	baz.AddArg(&bzl.LiteralExpr{Token: "z"})
 	baz.SetAttr("srcs", GlobValue{
 		Patterns: []string{"**"},
 		Excludes: []string{"*.pem"},
@@ -67,6 +71,7 @@ y_library(name = "bar")
 
 	got := strings.TrimSpace(string(f.Format()))
 	want := strings.TrimSpace(`
+load("//some:maybe.bzl", "maybe")
 load("b.bzl", "x_library")
 load(
     "c.bzl",
@@ -79,7 +84,8 @@ y_library(
     srcs = ["bar.y"],
 )
 
-z(
+maybe(
+    z,
     name = "baz",
     srcs = glob(
         ["**"],