add load_library: Return actual library object, rather than file path as find_library

[EricCousineau-TRI]

find_library shouldn't really return a path, but instead a loaded library.

This would permit things like ros2/rcutils#143 to not have to worry about paths, and could possibly reduce any possibility of ODR by returning currently loaded libraries rather than loading a new one.

[dirk-thomas]

find_library shouldn't really return a path, but instead a loaded library.

The current behavior of only finding the library makes sense for the existing function. That being said introducing another function like load_library would be an option.

[EricCousineau-TRI]

Aye, I didn't mean necessarily that find_library would keep it's name, so yeah, load_library would be good.

But I also wanted to imply that, at some point, it would be nice to fully encapsulate the path of a library and the only suggested workflow for handling libraries in ROS2 would be solely through load_library; then that could admit using already-loaded libraries. Then find_library would become deprecated (or at least discouraged...).