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. 

depends on https://github.com/hpcugent/vsc-base/pull/149


or a link that actually exists? (or is there a PR pending for it?)
does this link also have the policy wrt deprecating? (i.e. what does 'deprecated in `X.Y`' mean: code unusable in `X.Y` unless some magic commandline options is passed, will be removed in `X.Y+1`)