Move from github.com/eclipse/paho.golang to github.com/eclipse-paho/paho.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.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 library is still pre v1 I anticipate updating the module path at some point (to avoid future confusion).