Calling pkg_resources.fixup_namespace_packages() is only needed for packages
that use pkg_resources.declare_namespace() and hence they already imported
pkg_resources. When pkg_resources is not imported, we don't need to use it.
This avoids an unneeded runtime dependency on setuptools.
The code is tested by test_syspath_prepend_with_namespace_packages,
behavior should remain unchanged, hence no new test was added.
When people drop pkg_resources from sys.modules, they are on their own.
If someone has a actual use case making this valid to support,
they can come in and provide a test, a reference and a fix.
Miro Hrončok