Ok, so create three tar files:
1. base.tar, containing
simplistix/ simplistix/__init__.py
So this __init__.py can have code in it?
That's the point, yes.
And base.tar can have other modules and subpackages in it?
Certainly, yes.
What happens if the base and an addon both define a package called simplistix.somepackage?
Depends on whether simplistix.somepackage is a namespace package (it should). If so, they get merged just as any other namespace package.
2. addon1.tar, containing
simplistix/addon1.pth (containing a single "*")
What does that * mean?
See PEP 382 (search for "*").
I thought .pth files just had python in them?
Not at all - they never did. They have paths in them.
Unpack each of them anywhere on sys.path, in any order.
How would this work if base, addon1 and addon2 were eggs managed by buildout or setuptools?
What is a managed egg (i.e. what kind of management does buildout or setuptools apply to it)? Regards, Martin