fix get_view_key for meta-packages

[mathias-luedtke]

fixes #534

Should the stack keys get cached as well?

[mathias-luedtke]

Should get_loadable_views return meta-packages?

[mathias-luedtke]

in the last commit I have unified the handling of catkin packages. Feel free to drop it if you don't want to include this.

[k-okada]

great!!!, I found this solves our problem reporeted at #536

[mathias-luedtke]

@tfoote, @wjwwood: Do you have time to review and release it?
This bug is affecting the CI jobs of many repositories, given they contain meta-packages.

[wjwwood]

@ipa-mdl yup, I plan to do it today. Just juggling a few things at the moment.

[wjwwood]

@ipa-mdl I was really hoping to update the tests introduced into #531 to cover this case before merging this, but given the priority I think I'll go ahead and merge/release having tested it locally.

If you have time to extend those tests I would really appreciate it. It would help us avoid this in the future.

[mathias-luedtke]

Our CI tests are green again :D

If you have time to extend those tests I would really appreciate it. It would help us avoid this in the future.

I will try to.
At least a test for get_view_key should be added.
It would be great to extend the CLI tests in test_rosdep_main.py