performance: reduce the number of fills

- Remove unnecessary Fill() in Export()
- Change `set()` and the way we store outputs so we don't fill
  intermediaries as much
- WIP: Scan the tree only once. Changed LocalDirs to use cueflow rather than
  doing our own Walk. In a follow up we should use the same flow
  instance.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/client.go

@@ -162,10 +162,7 @@ func (c *Client) outputfn(ctx context.Context, r io.Reader) (*compiler.Value, er
 	lg := log.Ctx(ctx)
 
 	// FIXME: merge this into env output.
-	out, err := compiler.EmptyStruct()
-	if err != nil {
-		return nil, err
-	}
+	out := compiler.EmptyStruct()
 
 	tr := tar.NewReader(r)
 	for {