[FIX] delivery: speed up _compute_packages using read_group

Before this commit:
Packages were computed using a for loop, skipping any picking without
any packages.

After this commit:
Packages are computed using a read_group, speeding up the computed
method for large pickings with hundreds of stock move lines.

opw-3461706

closes odoo/odoo#134581

X-original-commit: 8dcad6e4
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>
Signed-off-by: Walravens Mathieu (wama) <wama@odoo.com>