Redirecting all stdout of subcommands to stderr (#2656)

go tool compile writes syntax errors to stdout, including the one reported in #2592. Meanwhile, all regular output of subcommands should have been written to files. We should direct all stdout of subcommands to stderr, so we can find all errors in stderr. This mirrors behavior in the go command

go/tools/builders/env.go

@@ -131,7 +131,9 @@ func (e *env) goCmd(cmd string, args ...string) []string {
 // environment from this process.
 func (e *env) runCommand(args []string) error {
 	cmd := exec.Command(args[0], args[1:]...)
-	cmd.Stdout = os.Stdout
+	// Redirecting stdout to stderr. This mirrors behavior in the go command:
+	// https://go.googlesource.com/go/+/refs/tags/go1.15.2/src/cmd/go/internal/work/exec.go#1958
+	cmd.Stdout = os.Stderr
 	cmd.Stderr = os.Stderr
 	return runAndLogCommand(cmd, e.verbose)
 }