Lazily import C extensions

[ivanpauno]

Yet another alternative to #417 #420.
See #420 (comment).

I ended up having a lot of problems with the qos module after applying lazy import, as all the default profiles are constructed at module level.
For working around the problem, I did a lot of changes that I don't like at all.

I would rather solve ros2/build_farmer#235 directly in launch_testing_ros, and figure out later how to avoid having to "hack" the imports while testing in rclpy (which is the root cause of ros2/build_farmer#235).

I would rather not merge this PR nor apply lazy import in general, as I believe is creating more problems than solving.

[ivanpauno]

This is the launch_testing_ros patch that @hidmic sent:
ros2/launch_ros#69

[ivanpauno]

Closing, because this is outdated.
We should still investigate ways of solving this issue.

[dirk-thomas]

We should still investigate ways of solving this issue.

@ivanpauno Is there any open ticket for this? If not, please create one and reference the previous approaches.

Also is the branch still needed?

[ivanpauno]

@ivanpauno Is there any open ticket for this? If not, please create one and reference the previous approaches.

See #560.

Also is the branch still needed?

I deleted the branch on one of the PRs, and forgot to do ot in the others.