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 don't understand the remark. the code doesn't do anything, so remove it. 
also, any tests related to deprecated behaviour are better of in a `deprecated_config.py` test. no need to complicate the tests with old code paths that will be removed soon-ish.