general: switch HPI to src/ layout

[karlicoss]

I'm thinking that would be nice to switch to src/ layout for consistency (and generally it tends to have less gotchas).
Should be smooth, the only concern is that it will break existing editable installations (since egg link would be pointing to /path/to/hpi_repo rather than /path/to/hpi_repo/src), unless the user reinstalls it.
I think that should be solvable just by putting a symlink from my to src/my though, and could add a warning suggesting the user to reinstall. Not sure if there are any other issues?
cc @seanbreckenridge

[purarue]

Yeah seems fine

One would need to git pull to update an editable install anyways, so they'd probably notice that

Will probably swap the HPI-template and my repo as well just for the consistency

[karlicoss]

Been experimenting more with editable installs etc, and seems like src/ layout works better in that case as well, it creates a simple .pth file in site-packages with a path to src/ dir, instead of dynamic __editable*.py madness, in particular this means less issues for running mypy with editable install python/mypy#13392 (comment)

So going to finally switch it soon