[FIX] stock: multiple orderpoint

- the oderpoint_id field on the purchase order line wasn't used to not
  merge the po lines
- if the orderpoint_id field is set, use the orderpoint location to
  create the move
- if multiple stock moves with different source locations go through
  `_merge_moves`, they should obviously NOT be merged
- `virtual_available` computed field was not being invalidated when the
  context keys "warehouse" and "location" changed

task-2001462

closes odoo/odoo#40573

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>