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 ;-)


no, it's there on purpose...

the default implementation of `extra_options` in `EasyBlock` must still return a list (of tuples) for now, until we switch to making it return a dict in EasyBuild v2.0

the list will be empty by default though, so I've added the check here to only complain in case the list is non-empty...