Add stock_helper: share common code used by stock modules

The first method is: StockLocation.is_sublocation_of()

This method is currently used in:

* wms/stock_dynamic_routing
* wms/shopfloor
* wms/stock_move_source_relocate
* stock-logistics-warehouse/stock_reserve_rule
* ddmrp/ddmrp

The goal will be to use this module as dependency instead of
reimplementing the method in each.

Other methods should follow in "stock_helper".

Note: I opened https://github.com/odoo/odoo/pull/53866 to propose a
generic version of this method, expecting odoo's opinion, but got no
answer.

setup/stock_helper/odoo/addons/stock_helper

@@ -0,0 +1 @@
+../../../../stock_helper

setup/stock_helper/setup.py

@@ -0,0 +1,6 @@
+import setuptools
+
+setuptools.setup(
+    setup_requires=['setuptools-odoo'],
+    odoo_addon=True,
+)