Fix packages when it doesn't include dirs with source

[thatch]

See python-packaging/highlighter#1

[thatch]

A common layout is one like this:

setup.py
the_package/
  __init__.py
  foo.py
  tests/
    __init__.py
    test_foo.py

If you specify

packages =
  the_package

it should be corrected to

packages =
  the_package
  the_package.tests

The crawling logic can probably use importall.finder.find_importable_names relative to package_dir from the config.

This is an intermediate issue, and some familiarity with packaging with setuptools is helpful.