add a "python package" builder, along with a "namespace package" and a "subpackage" builder.
refactor dexterity event tests to work well with the changed testing layer.

The testing layer now isolates the component registry, therefore the global site manager
should no longer be used in the test, using the local portal site manager instead.

Since we do isolate now, we no longer need to tear down component registration.
add a "genericsetup profile" builder for use within a package.
add a "plone upgrade step" builder.
upgrade step builder: add .calling(func), which serializes a callable.
generic setup profile builder: let default title be the package name.