Fix library import problems

I fixed these a while ago, but they reappeared in 1943f3278. I'm pretty certain people aren't following the instructions which dictate that scripts within Python libraries should be launched by using `python -m PATH.TO.SCRIPT` as suggested by Guido in the rejected PEP-3122. I've tried to clarify the documentation to assist in this. I don't want to go into why this is the "right" way to do it, as reading up on why PEP-3122 was rejected will take care of that.

@JoonasMelin Since your commit reintroduced this problem, can you review this and let me know if you can still use mavlink with these changes (assuming you add MAVLink to your PYTHONPATH)? I don't want to reintroduce the same bugs you were fighting back in July.

Looks like I have a dangling commit on my master branch, but it's benign and follows from other commits that I've had accepted recently, so shouldn't be controversial.