Build Error

In a workspace with other packages, a regular colcon build will result in an error:

```
Running Sphinx v1.6.7

Configuration error:
There is a programable error in your configuration file:

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/sphinx/config.py", line 157, in __init__
execfile_(filename, config)
File "/usr/lib/python2.7/dist-packages/sphinx/util/pycompat.py", line 150, in execfile_
exec_(code, _globals)
File "/usr/lib/python2.7/dist-packages/six.py", line 709, in exec_
exec("""exec _code_ in _globs_, _locs_""")
File "", line 1, in
File "conf.py", line 10, in
import catkin_pkg.package
ImportError: No module named catkin_pkg.package
```

Start by reproducing the regular colcon build in a workspace containing other packages, then inspect the conf.py entry point mentioned in the traceback and its import of catkin_pkg.package. Done means the documentation build no longer reports the shown ImportError during colcon build.

Written by the indexing model from the issue text.