Simplify the setup-py rules.

[benjyw]

The old code repeated some logic now centralized in python_sources.py and
elsewhere, notably the ancestor __init__.py discovery. We now just use that logic
instead of recapitulating it.

This required some changes to the helper function that discovers packages
and package data (for the purpose of writing those into the setup.py metadata).
That code is now simpler, and in particular no longer needs to know anything
about targets. This leads to a very slight behavior change: previously we would
infer the existence of a "python package" from the existence of resources, even when
no .py files were available. This is incorrect: to load a resources using pkgutil or
pkg_resources, they must be loaded relative to some actual Python package
(which by definition requires the presence of a .py file, __init__.py or other).
Now we handle this correctly - a nice case where the simpler code is also more correct.

This change also removes the check that injected missing __init__.py files must be
empty. This check was insufficient, e.g., an __init__.py containing a namespace
package invocation should also be fine to inject. And for setup.py this often must
be the case - if you have multiple dists in the same source tree, they must be in
a namespace package or the dists won't import correctly. Instead we just blindly
inject any ancestor __init__.py, and rely on the user that their __init__.py
hygiene is good. In particular, if an __init__.py has deps, injection won't do
anything special with those, so they will have to actually depend on the __init__.py,
either explicitly or via inference. In the setup-py case, however, ancestor
__init__.py files are almost certainly just to create namespace packages, as
any code in them would be embedded in multiple dists, which is a bad idea.

Since that empty file check is removed, the remaining code was too trivial to
exist on its own, so it was removed, and its one-line implementation moved to
the two call sites.

[ci skip-rust]

[ci skip-build-wheels]

[coveralls]

Coverage remained the same at 0.0% when pulling 5096356 on benjyw:setup_py_simplification into 685e51d on pantsbuild:master.

[Eric-Arellano]

Yay simplification!

I agree with the decision to no longer error if the __init__.py file has content in it.

[Eric-Arellano]

This was a bug, right?

[benjyw]

I think so (see description of this PR).

[benjyw]

Added a test for files(), which actually caught an issue with them. Fixed now, and will update the online docs appropriately once this is released.