refactor: use src/{package} layout

[mayeut]

This helps ensure what's being tested when installing the sdist or wheel and one wants to run tests on those installed packages on their own.

[henryiii]

Any thoughts, @jcfr or @thewtex? Otherwise I'd like to merge.

Personally, I'm already a huge fan of src layout, for the same reason as listed above; it's too easy to accidentally test the in-directory version instead of the installed version. src layout forces you to either use the packaging mechanisms or be completely intentional about running from a directory. This is really important if it is a built package.

[jcfr]

forces you to either use the packaging mechanisms or be completely intentional about running from a directory. This is really important if it is a built package.

Makes sense. Moving forward, I will adopt a similar structure for our projects.