TypedPython: embed a package-relative source path, not the host's absolute path #112

cgen embeds tp_source_path as the absolute host path of the module (it becomes the compiled code's filename in tracebacks). Two builds of the same source on different machines therefore produce different C and different extensions, and the build path leaks into the artefact. pypackpack's compile slot (pypackpack #33, Q4) relies on byte-identical outputs for the manifest SHA-256 used by code push.

Change: embed the module's path relative to the project/package root (app/physics/nbody.py).

Done when: a test builds the same module from two different absolute roots and the generated C is byte-identical; a traceback from interpreted fallback code names the relative path.