Clean some legacy files and move some build files (#23699)

* Clean the "tools" directory. The "tools" directory contains only two
files, move them.
* The "external_renderer.go" works like "cat" command to echo Stdin to
Stdout , to help testing.
* The `// gobuild: external_renderer` is incorrect, there should be no
space: `//gobuild: external_renderer`
* The `fmt.Print(os.Args[1])` is not a well-defined behavior, and it's
never used.
* The "watch.sh" is for "make watch", it's somewhat related to "build"
* After this PR, there is no "tools" directory, the project root
directory looks slightly simpler than before.
* Remove the legacy "contrib/autoboot.sh", there is no
"gogs_supervisord.sh"
* Remove the legacy "contrib/mysql.sql", it's never mentioned anywhere.
* Remove the legacy "contrib/pr/checkout.go", it has been broken for
long time, and it introduces unnecessary dependencies of the main code
base.

build/code-batch-process.go

@@ -65,7 +65,6 @@ func newFileCollector(fileFilter string, batchSize int) (*fileCollector, error)
 			"modules",
 			"routers",
 			"services",
-			"tools",
 		}
 		co.includePatterns = append(co.includePatterns, regexp.MustCompile(`.*\.go$`))