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 ;-)


We can't get rid of the old-style config file yet, since it still handles old-style configuring... The deprecation of the old-style variables etc. is handled in different places.

Without the if, using `--deprecated` triggers a deprecation error straight away, since `easybuild_config.py` is still there (and must be).

We can trigger a deprecation (error) if a non-default old-style config file was specified via `-C`.