Fix bug in parallel_walk, along with a few structural bugs that this fix revealed:

1. The conversion process was inconsistently packaging the final output into modules or lists. This CL uniformly uses a list of nodes as output from all *_to_graph functions. As a side effect, converter_testing.py asserts that the output is always a single node and extracts it, so there is no need for tests to unpack it any more. Modify the compiler to skip generating a source map by default.

2. The class converter was incorrectly saving the superclass value to the string 'object' instead of the symbol `object`.

Additional refactoring that was caught along: Simplify the source mapping code, move it to origin_info.py, add tests and additional checks. Slightly simplify the error rewriting mechanism.

PiperOrigin-RevId: 206087110