caddyfile: Introduce basic linting and fmt check (#3923)

* caddyfile: Introduce basic linting and fmt check

This will help encourage people to keep their Caddyfiles tidy.

* Remove unrelated tests

I am not sure that testing the output of warnings here is quite the
right idea; these tests are just for syntax and parsing success.

cmd/commandfuncs.go

@@ -463,17 +463,22 @@ func cmdAdaptConfig(fl Flags) (int, error) {
 			fmt.Errorf("reading input file: %v", err)
 	}
 
-	opts := make(map[string]interface{})
-	if adaptCmdPrettyFlag {
-		opts["pretty"] = "true"
-	}
-	opts["filename"] = adaptCmdInputFlag
+	opts := map[string]interface{}{"filename": adaptCmdInputFlag}
 
 	adaptedConfig, warnings, err := cfgAdapter.Adapt(input, opts)
 	if err != nil {
 		return caddy.ExitCodeFailedStartup, err
 	}
 
+	if adaptCmdPrettyFlag {
+		var prettyBuf bytes.Buffer
+		err = json.Indent(&prettyBuf, adaptedConfig, "", "\t")
+		if err != nil {
+			return caddy.ExitCodeFailedStartup, err
+		}
+		adaptedConfig = prettyBuf.Bytes()
+	}
+
 	// print warnings to stderr
 	for _, warn := range warnings {
 		msg := warn.Message