Revert "Port some of the generator API to cmGeneratorTarget."

This reverts commit d9c87ca822e78ccea41c004e62d39d387baf0790.
Revert "Order cmGeneratorTargets deterministically."

This reverts commit 8451e4d91310cefeec706b7f82d5ea1fec26c24b.
Merge topic 'use-generator-target' into next

70112e0 Revert "Order cmGeneratorTargets deterministically."
2283d65 Revert "Port some of the generator API to cmGeneratorTarget."
Add a null check to the generator target accessor.
Merge topic 'use-generator-target' into next

aa29e64 Add a null check to the generator target accessor.