We should not allow adding products to a package

Description

Steps to reproduce

1. Create a package in a Commerce catalog.
2. Go to its "Package Entries" table.
3. It does not allow you to drag and drop products. However, it's possible to create a new product as an item in a package.

Expected: When creating new items for a package, only variants and packages are allowed.

Actual: You can create a new product as an item in a package.