CMake integration with moo render-many

[brettviren]

A moo render-many determines the generated file names from the model itself.

In order for a build system like CMake to drive a moo render-many it must know:

• the names of files to be generated
• the name of the model file and any implicit dependencies
• the name of the template file and any implicit dependencies

This first one adds a new wrinkle. The latter two can already be discovered by moo imports and the moo.cmake support via CMake function moo_codegen() makes use of that.

What is then needed is something equivalent to moo imports that dumps the keys of a model that will be used in a moo render-many. And, this file needs to be dumpable in a CMake format like moo imports can do now.