Move from github.com/eclipse/paho.mqtt.golang to github.com/eclipse-paho/paho.mqtt.golang

[MattBrittan]

Eclipse is reogranising repos to move projects off the main github.com/eclipse repo. This repo is now github.com/eclipse-paho/paho.mqtt.golang. Github redirects should, hopefully, mean that this change has no impact on users, but I am raising this issue in case problems do arise (please add comments).

Following the change I have completed some basic tests and everything appears to be working as expected.

As this is a mature library I have no plans to change the module path (my understanding is that the redirect wil remain in place indefinitly).