Add explicit deps from manage.py to settings.py in each app.

[benjyw]

With this change, the example commands in the README work properly
(e.g., pants run helloworld/service/frontend/manage.py -- runserver)

Also explicitly note the BUILDROOT, now that ./pants is no more.

[jsirois]

Although this LGTM on its own, it still seems like maybe a bug that running a main.py directly that is owned by a pex_binary doesn't go through that pex_binary.

[benjyw]

Even if it did go through the pex_binary, that doesn't solve the fundamental issue, it only happens to work in this case because there is a handwritten dep from the pex_binary on all of :lib, which happens to include settings.py.

So I think the issues are orthogonal - manage.py depends on settings.py regardless of any pex_binary, and that dep can't be inferred, so should be written manually.

[jsirois]

Yeah, agreed. The not using an "owning" pex_binary is an orthogonal bug. but it does seem a bug on its own - you set pex_binary knobs (other than dependencies say), and they get ignored. Here it caused effectively a Pants upgrade break, which revealed a better way to structure the dependencies in python_source instead of in pex_binary.

[benjyw]

Well, but you can still run the pex_binary to get that. But if you run the source then A) that is uniform behavior with running sources that don't have a pex_binary, and B) it is a lot faster.

[jsirois]

But if the pex_binary settings are required for the thing to run at all, say venv_site_packages_copies=True which is required for some known real cases in the world, you get a failure for no reason. It's a strange arrangement that you set up a pex_binary to work, and then running it only works by using the target address. Using the file address just has to be known to fail.