exp: hide experiments cmd name

[jorgeorpinel]

exp is an alias of experiments (or vice versa) but mainly we use exp everywhere so let's hide experiments (even if it exists in core DVC.

Separate question: what to do with the long form command in DVC core cc @dberenbaum

[dberenbaum]

I don't see much harm in keeping as is (I haven't noticed that it's either causing confusion or additional maintenance burden). I get wanting to clean things up, but I wouldn't prioritize any changes in DVC unless you or @shcheklein think it's needed.