Switch to src layout instead of flat layout

[parthea]

With the migration to native namespace packages, editable installs may not work due to known limitations. We can reduce the number of issues by switching to a src layout instead of a flat layout, as suggested in the docs.

Support for PEP 420-style implicit namespace packages for projects structured using flat-layout is still experimental. If you experience problems, you can try converting your package structure to the src-layout.