[ADD] product_main_seller: Restore OCA addon from original version

- Restored product_main_seller addon to fix dependency chain
- Addon required by product_origin_char and website_sale_aplicoop

[IMP] website_sale_aplicoop: Add demo data configuration to manifest

- Updated __manifest__.py to include demo data files
- Demo data includes partners, suppliers, products, group orders, and sale orders
- Demo data successfully loads during module installation

product_main_seller/models/__init__.py

@@ -0,0 +1 @@
+from . import product_template

product_main_seller/models/product_template.py

@@ -0,0 +1,31 @@
+# Copyright (C) 2022 - Today: GRAP (http://www.grap.coop)
+# @author: Quentin DUPONT (quentin.dupont@grap.coop)
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
+
+from odoo import api
+from odoo import fields
+from odoo import models
+
+
+class ProductTemplate(models.Model):
+    _inherit = "product.template"
+
+    main_seller_id = fields.Many2one(
+        comodel_name="res.partner",
+        string="Main Vendor",
+        help="Put your supplier info in first position to set as main vendor",
+        compute="_compute_main_seller_id",
+        store=True,
+    )
+
+    @api.depends("variant_seller_ids.sequence", "variant_seller_ids.partner_id.active")
+    def _compute_main_seller_id(self):
+        for template in self:
+            if template.variant_seller_ids:
+                template.main_seller_id = fields.first(
+                    template.variant_seller_ids.filtered(
+                        lambda seller: seller.partner_id.active
+                    )
+                ).partner_id
+            else:
+                template.main_seller_id = False