refactor: move pkg/utf7 to internal/utf7

internal/utf7/utf7.go

@@ -0,0 +1,34 @@
+// Package utf7 implements modified UTF-7 encoding defined in RFC 3501 section 5.1.3
+package utf7
+
+import (
+	"encoding/base64"
+
+	"golang.org/x/text/encoding"
+)
+
+const (
+	min = 0x20 // Minimum self-representing UTF-7 value
+	max = 0x7E // Maximum self-representing UTF-7 value
+
+	repl = '\uFFFD' // Unicode replacement code point
+)
+
+var b64Enc = base64.NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,")
+
+type enc struct{}
+
+func (e enc) NewDecoder() *encoding.Decoder {
+	return &encoding.Decoder{
+		Transformer: &decoder{true},
+	}
+}
+
+func (e enc) NewEncoder() *encoding.Encoder {
+	return &encoding.Encoder{
+		Transformer: &encoder{},
+	}
+}
+
+// Encoding is the modified UTF-7 encoding.
+var Encoding encoding.Encoding = enc{}