Remove 'Namespace' from qualified constraints.

This commit changes the meaning of constraints like 'pkg > 5'. Previously,
'pkg > 5' only applied to the top-level 'pkg' goal without a 'Namespace', but
now it applies to any top-level goal for 'pkg'. However, the 'Namespace' field is
currently only used by --independent-goals, so this change has no effect on
behavior.