Issue/stable/pup 2158 hide dir envs behind feature flag

This just adds additional test coverage and changes on top of PR2512.  One issue which Eric and I were noodling over @zaphod42, is that the new static configured environment is listed when enumerating environments, unless there is a directory environment for it.  We were having difficulty deciding whether this is a good thing or a bad thing, but have decided to leave it as is for the present.  Did you have any thoughts about this?  It would be fairly simply to keep from enumerating, I think, with, say, a StaticUnlisted < Static loader that overloads list() { [] } (Or StaticPrivate maybe?).

I guess from the perspective of enumerable environments as a service, this configured environment is not something an agent could use effectively, and so should be suppressed?  To go against what I just said above, about leaving it in...