Remove legacy unknwon/com package (#19298)

Follows: #19284
* The `CopyDir` is only used inside test code
* Rewrite `ToSnakeCase` with more test cases
* The `RedisCacher` only put strings into cache, here we use internal `toStr` to replace the legacy `ToStr`
* The `UniqueQueue` can use string as ID directly, no need to call `ToStr`

modules/util/string.go

@@ -0,0 +1,88 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package util
+
+import "github.com/yuin/goldmark/util"
+
+func isSnakeCaseUpper(c byte) bool {
+	return 'A' <= c && c <= 'Z'
+}
+
+func isSnakeCaseLowerOrNumber(c byte) bool {
+	return 'a' <= c && c <= 'z' || '0' <= c && c <= '9'
+}
+
+// ToSnakeCase convert the input string to snake_case format.
+//
+// Some samples.
+//     "FirstName"  => "first_name"
+//     "HTTPServer" => "http_server"
+//     "NoHTTPS"    => "no_https"
+//     "GO_PATH"    => "go_path"
+//     "GO PATH"    => "go_path"      // space is converted to underscore.
+//     "GO-PATH"    => "go_path"      // hyphen is converted to underscore.
+//
+func ToSnakeCase(input string) string {
+	if len(input) == 0 {
+		return ""
+	}
+
+	var res []byte
+	if len(input) == 1 {
+		c := input[0]
+		if isSnakeCaseUpper(c) {
+			res = []byte{c + 'a' - 'A'}
+		} else if isSnakeCaseLowerOrNumber(c) {
+			res = []byte{c}
+		} else {
+			res = []byte{'_'}
+		}
+	} else {
+		res = make([]byte, 0, len(input)*4/3)
+		pos := 0
+		needSep := false
+		for pos < len(input) {
+			c := input[pos]
+			if c >= 0x80 {
+				res = append(res, c)
+				pos++
+				continue
+			}
+			isUpper := isSnakeCaseUpper(c)
+			if isUpper || isSnakeCaseLowerOrNumber(c) {
+				end := pos + 1
+				if isUpper {
+					// skip the following upper letters
+					for end < len(input) && isSnakeCaseUpper(input[end]) {
+						end++
+					}
+					if end-pos > 1 && end < len(input) && isSnakeCaseLowerOrNumber(input[end]) {
+						end--
+					}
+				}
+				// skip the following lower or number letters
+				for end < len(input) && (isSnakeCaseLowerOrNumber(input[end]) || input[end] >= 0x80) {
+					end++
+				}
+				if needSep {
+					res = append(res, '_')
+				}
+				res = append(res, input[pos:end]...)
+				pos = end
+				needSep = true
+			} else {
+				res = append(res, '_')
+				pos++
+				needSep = false
+			}
+		}
+		for i := 0; i < len(res); i++ {
+			if isSnakeCaseUpper(res[i]) {
+				res[i] += 'a' - 'A'
+			}
+		}
+	}
+	return util.BytesToReadOnlyString(res)
+}