Fix trailing slash handling in pkg_resources.ZipProvider

Given a ZipProvider, if the underlying ZipImporter prefix is empty,
then resource_listdir('') and resource_listdir('subdir/') fail, while
resource_listdir('/') and resource_listdir('subdir') succeed.

On the other hand, if the underlying ZipImport prefix is not empty,
then resource_listdir('/') fails but resource_listdir('') succeeds.

With this change, the cases listed succeed with or without trailing
slashes.

fix `data_files` handling when installing from wheel

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.