[v2] Refactor `AssetManager` about assets

[promatik]

This cases are also covered by bassets 🎉

[tabacitu]

Awesome. I'd still keep this open for one more week, until we move everything to backpack/bassets. It was about code architecture, and my spidey sense does say that the current code is a bit messy, and we can improve it by creating a Basset object, and a Bassets collection where we store those objects. But I'm not 100% sure, I'll know more after the code is in a separate repo... we can do a code quality check after we've fixed all bugs.