Forum OpenACS Q&A: Re: I don't get it - new to acs
Here's some explanation about the old and new package structure. I don't think there's any reason why packages from ACS3 couldn't be ported, they just don't work as they are because of the fundamental changes in the system.
OpenACS, like any system of its kind, is big and you probably can't get the big picture by just looking the code. You can't get OpenACS without any packages, since everything is organized into them. You can get the bare minimum of OpenACS by checking it out with CVS with module name "acs-core" - see CVS instructions for how to do this.