internal/third_party/yaml: drop support for non-standard scalars

CUE uses a mix of YAML packages. The older one in third_party/yaml
recognizes boolean scalars such as `y` and `n` that the newer package
does not quote, so round-tripping fails when those unquoted strings
get converted to booleans.

Fix this (pending replacement of the whole package) by removing support for those scalars.

Fixes cuelang/cue#512

Change-Id: I3958664a24a3191f0d3004b259ae26d2db86d0b5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7161
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/testdata/script/issue512.txt

@@ -0,0 +1,23 @@
+cue export -o bools.yaml bools.cue
+cue import -o bools2.cue bools.yaml
+cmp bools.cue bools2.cue
+
+-- bools.cue --
+x: [
+	"n",
+	"N",
+	"no",
+	"No",
+	"NO",
+	"y",
+	"Y",
+	"yes",
+	"Yes",
+	"YES",
+	"off",
+	"Off",
+	"OFF",
+	"on",
+	"On",
+	"ON",
+]

cmd/cue/cmd/testdata/script/vet_file.txt

@@ -31,7 +31,7 @@ translations:
 ---
 translations:
   hello:
-    lang: no
+    lang: false
     text: Hallo
 ---
 translations:

internal/third_party/yaml/decode_test.go

@@ -100,16 +100,22 @@ var unmarshalTests = []struct {
 	// Bools from spec
 	{
 		"canonical: y",
-		"canonical: true",
+		`canonical: "y"`,
+	}, {
+		"answer: n",
+		`answer: "n"`,
 	}, {
 		"answer: NO",
-		"answer: false",
+		`answer: "NO"`,
 	}, {
 		"logical: True",
 		"logical: true",
 	}, {
 		"option: on",
-		"option: true",
+		`option: "on"`,
+	}, {
+		"answer: off",
+		`answer: "off"`,
 	},
 	// Ints from spec
 	{
@@ -243,7 +249,7 @@ apple: "newline"`,
 		"a: [1, 2]",
 	}, {
 		"a: y",
-		"a: true",
+		`a: "y"`,
 	}, {
 		"{ a: 1, b: {c: 1} }",
 		`a: 1, b: {c: 1}`,

internal/third_party/yaml/resolve.go

@@ -24,7 +24,7 @@ func init() {
 	for _, c := range "0123456789" {
 		t[int(c)] = 'D' // Digit
 	}
-	for _, c := range "yYnNtTfFoO~" {
+	for _, c := range "nNtTfF~" {
 		t[int(c)] = 'M' // In map
 	}
 	t[int('.')] = '.' // Float (potentially in map)
@@ -34,12 +34,8 @@ func init() {
 		tag string
 		l   []string
 	}{
-		{true, yaml_BOOL_TAG, []string{"y", "Y", "yes", "Yes", "YES"}},
 		{true, yaml_BOOL_TAG, []string{"true", "True", "TRUE"}},
-		{true, yaml_BOOL_TAG, []string{"on", "On", "ON"}},
-		{false, yaml_BOOL_TAG, []string{"n", "N", "no", "No", "NO"}},
 		{false, yaml_BOOL_TAG, []string{"false", "False", "FALSE"}},
-		{false, yaml_BOOL_TAG, []string{"off", "Off", "OFF"}},
 		{nil, yaml_NULL_TAG, []string{"", "~", "null", "Null", "NULL"}},
 		{math.NaN(), yaml_FLOAT_TAG, []string{".nan", ".NaN", ".NAN"}},
 		{math.Inf(+1), yaml_FLOAT_TAG, []string{".inf", ".Inf", ".INF"}},