[FEATURE] pack_line_ids on product.template use as default if none are set on product.product

[tarteo]

Is your feature request related to a problem?
Currently if you have a product.template with variants (product.product). You will have to set the pack lines for every product even if they're all the same for all variants.

product-pack/product_pack/models/product_template.py

Lines 35 to 37 in 66b5edb

	pack_line_ids = fields.One2many(
	related="product_variant_ids.pack_line_ids",
	)

product-pack/product_pack/views/product_template_views.xml

Lines 50 to 54 in 66b5edb

	<page name="page_pack" position="attributes">
	<attribute
	name="attrs"
	>{'invisible': ['|', ('product_variant_count', '>', 1), ('pack_ok', '=', False)]}</attribute>
	</page>

Describe the solution you'd like
Add a field product_tmpl_id to product.pack.line and make it work kinda like product.supplierinfo. Where it falls back to product.template or make product.pack.line linked to a product.product override the ones on product.template.

Additional context
We have a product which has many variants but always is packed with let say four bolts and screws (just an theoretical example).