Add new argument `package_adder` to `DatafileHandler.assemble()`

[JonoYang]

We want to reuse the code from the assembly() method of the different PackageHandlers in scancode.io, and the current method of associating Packages to Resources (in the assembly() methods), where we append package_uid to resource.for_packages, does not work there. We need to be able to pass in a function that performs the Package - Resource association, so we can use the assembly() methods in different contexts.

The package_adder argument is a function that takes in a package_uid, resource, and codebase. It is used in places where we want to associate a Resource to a Package using the package_uid.

[AyanSinhaMahapatra]

This was added with #3035, closing!
Thanks @JonoYang!