py: Implement __delitem__ method for classes.
builtinimport: Add basic support for namespace packages.

That was easy - just avoid erroring out on seeing candidate dir for namespace
package. That's far from being complete though - namespace packages should
support importing portions of package from different sys.path entries, here
we require first matching entry to contain all namespace package's portions.

And yet, that's a way to put parts of the same Python package into multiple
installable package - something we really need for *Micro*Python.