cmGeneratorTarget: refactor header set verification to collect then create

Separate the collection of verification stubs from the creation of the
verification target.  Previously, the target was created lazily on the
first stub found, with subsequent stubs added incrementally.  Now, all
stubs are collected into a vector first, and the target is created
afterward with all sources at once.

No behavior change; this is a pure refactoring to prepare for creating
targets even when no stubs are found.