internal/pkg: streaming archive reader/writer

This is much more robust and efficient than the simple buffering implementation for larger files. Allocations happen almost exclusively in WalkDir.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -595,7 +595,7 @@ func main() {
 							0400,
 						); err != nil {
 							return err
-						} else if _, err = pkg.Flatten(
+						} else if err = pkg.Write(
 							os.DirFS(pathname.String()),
 							".",
 							f,