addrs: Fix LegacyString for builtin provider addrs

Builtin provider addrs (i.e. "terraform.io/builtin/terraform") should be
able to convert to legacy string form (i.e. "terraform"). This ensures
that we can safely round-trip through ParseLegacyAbsProviderConfig,
which can return either a legacy or a builtin provider addr.

addrs/provider.go

@@ -156,8 +156,8 @@ func (pt Provider) LegacyString() string {
 	if pt.IsZero() {
 		panic("called LegacyString on zero-value addrs.Provider")
 	}
-	if pt.Namespace != LegacyProviderNamespace {
-		panic(pt.String() + " is not a legacy addrs.Provider")
+	if pt.Namespace != LegacyProviderNamespace && pt.Namespace != BuiltInProviderNamespace {
+		panic(pt.String() + " cannot be represented as a legacy string")
 	}
 	return pt.Type
 }

addrs/provider_test.go

@@ -54,6 +54,37 @@ func TestProviderString(t *testing.T) {
 	}
 }
 
+func TestProviderLegacyString(t *testing.T) {
+	tests := []struct {
+		Input Provider
+		Want  string
+	}{
+		{
+			Provider{
+				Type:      "test",
+				Hostname:  DefaultRegistryHost,
+				Namespace: LegacyProviderNamespace,
+			},
+			"test",
+		},
+		{
+			Provider{
+				Type:      "terraform",
+				Hostname:  BuiltInProviderHost,
+				Namespace: BuiltInProviderNamespace,
+			},
+			"terraform",
+		},
+	}
+
+	for _, test := range tests {
+		got := test.Input.LegacyString()
+		if got != test.Want {
+			t.Errorf("wrong result for %s\ngot:  %s\nwant: %s", test.Input.String(), got, test.Want)
+		}
+	}
+}
+
 func TestProviderDisplay(t *testing.T) {
 	tests := []struct {
 		Input Provider