Switch to native namespace (PEP 420)

[gforcada]

Part of plone/Products.CMFPlone#3928

This is a draft PR to switch to:

• src layout: https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/
• move to native namespaces: https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages
• uses a Plone 7.0 trove classifier to ensure that it can not be pushed to PyPI 🩹

[mister-roboto]

@gforcada thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!