KeyError with SourceDependencyWalker

[Mike Purvis]

Seeing these in our builds since updating to the current source master:

  File "env/local/lib/python2.7/site-packages/rosinstall_generator/distro.py", line 87, in get_recursive_dependencies
    dependencies |= walker.get_recursive_depends(pkg_name, ['buildtool', 'build', 'run', 'test'], ros_packages_only=True, ignore_pkgs=dependencies | excludes, limit_depth=limit_depth)
  File "env/local/lib/python2.7/site-packages/rosdistro/dependency_walker.py", line 78, in get_recursive_depends
    deps = self.get_depends(next_pkg_to_check, depend_type, ros_packages_only=ros_packages_only)
  File "env/local/lib/python2.7/site-packages/rosdistro/dependency_walker.py", line 61, in get_depends
    deps = self._get_dependencies(pkg_name, depend_type)
  File "env/local/lib/python2.7/site-packages/rosdistro/dependency_walker.py", line 120, in _get_dependencies
    pkg = self._get_package(pkg_name)
  File "env/local/lib/python2.7/site-packages/rosdistro/dependency_walker.py", line 133, in _get_package
    repo = self._distribution_instance.repositories[self._distribution_instance.source_packages[pkg_name].repository_name].source_repository
KeyError: 'package_which_definitely_does_exist'

@dirk-thomas I should have a patch for you shortly; just going through the delta between https://github.com/mikepurvis/rosdistro-1/releases/tag/0.4.7%2Bmpu1 and the current master to figure out where things went wrong.

(To clarify, there's no regression from current functionality, since SourceDependencyWalker is only used with a rosdistro which has a source cache...)