codecov does not include submodule code in the report or statistics

Relevant codecov.io threads:
- https://community.codecov.com/t/codecov-io-ui-not-resolving-submodule-paths-in-github/713
- https://community.codecov.com/t/problem-getting-the-source-code-from-your-provider-unable-to-show-line-by-line-coverage/4128
- https://community.codecov.com/t/coverage-of-external-files/1315

I have tried some variants in branch dev.ej/codecov-with-submodules to attempt to fix this, but no luck.

When I expect the payload uploaded to codecov, I can see that the submodule code is in the XML part of the payload, but the file list at the beginning does not include the submodule python files, which is probably why they're excluded. I've inspected the code in our upload action, https://github.com/codecov/codecov-action but I didn't find where the file list is being built or how to trigger the change we need.

Start by reviewing the codecov upload action and the attempted branch dev.ej/codecov-with-submodules, then compare the uploaded XML with the file list that precedes it. Trace where the file list is assembled and verify that submodule Python files appear in Codecov reports and statistics.

Written by the indexing model from the issue text.