only trigger deprecated warnings on using functionality which is actually deprecated

This contains changes required such that deprecation log messages are only hit when actual deprecated functionality is being relied upon.

This is necessary to make `--deprecated` useful, with the upcoming EasyBuild v2.0 release in mind. 

not finished yet, but I need this PR open before the feature freeze end of today ;-)


I'm just saying that this line should be uncommented, it's a mistake to comment it out

I'll look into fleshing out all tests w.r.t. deprecated behaviour into it's own test module