Add package_adder argument to assemble() #3034

[JonoYang]

This PR adds a new argument, package_adder, to the DatafileHandler.assemble() method and to its subclassed variants as well. This is done to address the issues of running the different assemble() methods in a context outside of scancode-toolkit, see #3034 and aboutcode-org/scancode.io#476.

[pombredanne]

Very nicely done! Clean and neat... one minor nitpicking: what about adding a test on one assemble function that passes a custom adder to validate the behaviour?
This would be nice... but this is good to merge as it is anyway.

[pombredanne]

We still need a CHANGELOG entry too.

[JonoYang]

@pombredanne Thanks for the review! I've added a new test where I run get_package_and_deps with a custom package adder and updated the changelog.

[pombredanne]

LGTM!