Py runtime: Move to relative imports

[pelson]

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This also fixes a few from antlr4 import *, which is considered bad practice unless being done to expose an interface (https://peps.python.org/pep-0008/#imports)

My changes seem to modify generated files in antlr4.xpath. I didn't yet look at fixing the generator.

There remains some significant issues withe the namespaces being exposed in the runtime - I would be happy to follow up after this change to tighten the names being made publicly available.