fix namespace packages handling of wheels

easy_install: add support for installing from wheels

Note: wheels are installed as eggs, so each install is self-contained
and multiple versions of the same package can be installed at the same
time.

Limitations:
- headers are not supported
- resulting egg metadata requirements have their markers stripped

Fixes #1208: Improved exception message of pkg_resources.ResolutionError

Add another test demonstrating that if requirements are declared in a non-deterministic order, they may appear in the metadata in non-deterministic order. Ref #458.

use ssl.create_default_context and SNI if available