Build script now in the repo

[brettcannon]

To help keep things moving (and anyone who works on this at the core dev sprints), I have added a relocatable.py script to this repo. The idea is that can help test approaches to building a relocatable build as well as make it easier to run the test suite. That way it should be easier to figure out what needs to be upstreamed as well as figuring out what approach to take in making this happen.

As well, I loosened the goals of this project such that needing an installer to do any patching is acceptable (I think even uv touches up some spots in python-build-standalone, so it seemed overly audacious to require no files gets touched upon installation, although if we can make that work without an install step that would be great!).