pbr should support package_dir

Our python packages locate source under the src directory instead of under the root directory. This is no problem for setuputils, since it provides the package_dir option. Example:

setup(
    ...
    packages=find_packages('src'),
    package_dir={'': 'src'},
    ...
)

Please add support to pbr for alternative source organization conventions like src that are supported by package_dir. Current behavior of pbr is to include the src dir and everything under it (leading to incorrect package paths).