Switch to src package directory layout

[instification]

Possible solution for #175

This change currently breaks the tests with the following errors:

Test-module import failures:

Module: src.plone.namedfile.tests.test_adapters

ModuleNotFoundError: No module named 'src'

...

Presumably there needs to be further changes to tox.ini or the tests to get them working in src layout

[mister-roboto]

@instification 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!

[instification]

Not needed see #175