groot/cmd/root-gen-streamer: use an in-memory buffer for generation

This CL modifies root-gen-streamer to use an in-memory buffer as the
destination of the code generation.
This is needed because the Go-1.19.10 version of x/go/packages.Load will
choke on an empty "output_gen.go" file.
This was the case as the old version root-gen-streamer was creating the
destination file for the code generation before calling the streamer
generation.

Signed-off-by: Sebastien Binet <binet@cern.ch>

groot/cmd/root-gen-streamer/main.go

@@ -6,6 +6,7 @@
 package main // import "go-hep.org/x/hep/groot/cmd/root-gen-streamer"
 
 import (
+	"bytes"
 	"flag"
 	"fmt"
 	"io"
@@ -54,8 +55,14 @@ options:
 	var (
 		err error
 		out io.WriteCloser
+		buf = new(bytes.Buffer)
 	)
 
+	err = generate(buf, *pkgPath, types)
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	switch *output {
 	case "":
 		out = os.Stdout
@@ -67,7 +74,7 @@ options:
 		defer out.Close()
 	}
 
-	err = generate(out, *pkgPath, types)
+	_, err = io.Copy(out, buf)
 	if err != nil {
 		log.Fatal(err)
 	}