Refine esky support in setup.py

* Add HAS_ESKY flag based on import tests rather than command line
  arguments.
* Add comments on reasoning for some setup kwargs
* Fix append bug for wmi dependency
* Revert part of e70d70aa75fedb90a6ef774a0131e0f3f2b6e516. Esky uses
  those kwargs regardless of the distribution stack used.
* Remove unused 'libraries' variable


This should fix #2986. This will restore the distutils warnings about unknown setup kwargs.