build: Fix parallel build of layers

Change layer dependencies to prevent collisions of scripts creating headers.
Use dependency on one custom target for each group of headers built by a custom command.
The layers json targets need to depend on the layers targets.
They expect to copy into a directory created by those targets.
Make dependency of vk_safe_struct.cpp indirect through generate_vk_layer_helpers.
That will wait for a single build of the generated files.

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