Move test for namespace-packages to test/import.

They are testing import-behaviour, don't they?
No longer base NamespaceImportDirector on BuiltinImportDirector.

I did this, since namespace packages without __init__-file are
represented as "built-in" in Python 2.7, e.g.:
  >>> sys.modules['zope']
  <module 'zope' (built-in)>
Remove test-case for namespace packages using pkgutil.extend_path().

We do not differentiate between these and those using
pkg_resources.declare_namespace: both have __init__-files.
Add mini-packages and script for 'nspkg2' namespace packages.
Cleanup build script.
Fix: build script contains hard-coded Python version.
Fix: build script does not remove destination eggs for nspkg2_*.egg.
Fix some paths in build script.
Make build script less verbose, so errors can be seen easier.
Fix: build-script does not copy nspkg2*.egg if it is a zip-file.
Change one of the nspkg2 eggs into a zipped one.

We need to test namespaces in zipped eggs, too.
Replace old (and wrong) nspkg2 test-package by new eggs.

Also add the eggs to sys.path in test_nspkg2 .
Add test-case for zipped namespace eggs: contains *only* a namespace-mod.
Add two more test-cases for zipped namespace eggs.

- namespace package with some real package below
- namespace package without any real package below