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 could reimplement what the old-style default config file `easybuild_config.py` does in `config.py`, but that would be wasted effort since we actually want to get rid of it

the deprecation of what is being defined via `easybuild_config.py` is handled in other places, in code blocks that can simply be dropped once we clean things up (EasyBuild v2.1?)

note that this whole block will disappear too (anything under the `if SUPPORT_OLDSTYLE`), so with EasyBuild v2.1 we will be able to just delete all of this, no need to convert code at all...