Add python package builders

This adds some additional default builders:

- a "zcml" builder for creating ZCML files
- a "python package" builder for creating a python package on the file system
- a "namespace package" builder, used internally by the "python package" builder
- a "subpackage" builder for extending a python package with nested packages
- a "genericsetup profile" builder
- a "plone upgrade step" builder for building Generic Setup upgrade steps

This was built in relation to the ftw.upgrade test refactorings.
Using builders instead of static code / GS profiles makes the tests more explicit and understandable and reduces testing layer code. The ftw.upgrade PR is coming soon..

/cc @maethu