These are great questions.
You are correct that -e is useful for testing code you are working on. That’s included in chapter 13, “Debugging Test Failures”, under section “Installing Cards in Editable Mode”.
Regarding testing code that is internal to a package and not exposed outside the package, see chapter 12, “Testing Scripts and Applications”, under section “Separating Code into src and tests Directories”.
However, short answer is use pythonpath setting. See API Reference — pytest documentation