follow package_dir automatically when set in the distribution

[thrau]

Fixes #16.

When package_dir is set in a setup.cfg like this:

[options]
package_dir=
    =localstack-core/
packages=find:

Then the Distribution object will have an attribute dist.package_dir that's a weird looking dict:

{"": "localstack-core"/}

There's also a way to have multiple package source dirs, but I don't know how to support that right now.