Improve coverage reporting

Add cov.pth file and COVERAGE_PROCESS_START to track coverage of subprocesses and e2e tests
e2e tests set COVERAGE_PROCESS_START and run command_pre hooks
Configure mapping of file paths for coverage
Add step to combine coverage results in GHA
Use plain coverage instead of pytest-cov. The pytest plugin interfers with parallel coverage collection.
Fix test_dependencies to actually run tests

python-wheel-build / fromager