Incomplete support for packages_root field in setup.cfg

Indeed, it does not seems to work out of the box.

I tried:

[files]
packages_root = src
files = src # or src/mypackage

It does not work. Would be cool to allow packages to be put into a subdirectory "src", without changing the name of the package and the way to access to it

also, it allows more easily package transitions:
In my package, I have a src/mypackage package. Now, let's say I have some method I want to move to another lib "anotherlib". I can temporarily create an src/anotherlib", time until pypi receive my new lib 'anotherlib', and I do not have code to change in python files.

Putting the code to src/ folder looks a good practice, and it is sad that I can't use pbr for such project structure :-(.

Steps to reproduce:

```
mkdir simplepkg/src/simplepkg
cd simplepkg
git init
```

src/simplepkg/__init__.py:
```
print("Hello, world!")
```

setup.py:
```
#!/usr/bin/env python

from setuptools import setup

setup(
    setup_requires=['pbr'],
    pbr=True,
)
```

setup.cfg:

```
[metadata]
name = simplepkg

[files]
packages_root = src/
packages =
    simplepkg
```

Create virtualenv and install our package:

```
python -m venv env
source env/bin/activate
pip install .
python -c 'import simplepkg'
```

The output is:

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'simplepkg'
```

I have noticed this also. If you run a 'pip install .' in the git checked out directory it doesn't work...but if you run 'python setup.py install' it does work. It happens at. Low level as I put debug statements in setuptools, distribute, and pbr. And dist itself finds no packages with 'has_pure_python'. And indeed it finds no packages in general. I'll perform more debugging. This is high on my list because I want the other machinery provided by pbr.

Sorry for the terrible description above. This is still a real problem. But I found a way to still get the functionality of pbr to work while still using setuptools properly.

Put the `package_dir={'':'src/'} in the setup.py (this is needed as per a warning on https://setuptools.readthedocs.io/en/latest/setuptools.html#configuring-setup-using-setup-cfg-files under "Using find_packages".

Put an [options] section in setup.cfg since setuptools supports setup.cfg style config now. Specify
[options]
package_dir = src/
packages = boardgame_framework

SetupTools will accept that info and still play well with pbr. Stop using [files] section of pbr.

You still get all the autogeneration goodness of pbr with authors, changelog, version number, release notes, etc....and you get proper sdist and installs!

I plan to keep looking at the behavior in pbr to see if I can fix it but I'm putting this info here in case a "real" pbr developer can connect the dots on what info is not being forwarded around properly.