Improve how rclpy libraries are imported

[ivanpauno]

Feature request

Feature description

Currently, it's impossible to import rclpy C extensions at Module level.
That's because at test time they are being imported in a different way (1, 2).

Implementation considerations

Approaches tried in the past to solve the problem:

• Lazily import C extensions #422
• Avoid the limitation of not being able to import rclpy C extensions at module level #417
• Avoid the limitation of not being able to import rclpy C extensions at module level [alternative] #420

Other approaches:

• Install all tests in build folder, and run tests there.

[dirk-thomas]

I good use case which should be supported (without needing to source the setup file): #561.