Fix improper XHPAST parsing of namespace grammar like "use x as private;"

Summary:
Depends on D21067. Ref T13492. Converting unit tests to be readable exposed this error in the grammar.

Normally, "grammar_rule" rules emit a standalone Node. In this case, the bottom-level grammar rule is a collection of trivial rules and the callers configure the Node. Wrap the bottom-level rule in a configuration rule so the node is configured correctly and consistently, in exactly one place.

Test Plan: Ran unit tests.

Maniphest Tasks: T13492

Differential Revision: https://secure.phabricator.com/D21068