Split CreateGeneratorTargets into two methods.

As the generate-time-related API is moving to cmGeneratorTarget, almost
all of generation code needs to be able to access instances of it.
Move GenerateTargetManifest to cmGeneratorTarget.
Move TraceDependencies to cmGeneratorTarget.
Merge topic 'use-generator-target' into next

5886d98 Move TraceDependencies to cmGeneratorTarget.
90ef1cf Move GenerateTargetManifest to cmGeneratorTarget.
25f1df3 Split CreateGeneratorTargets into two methods.