Split the build-time code out of compiler_gym package.

[ChrisCummins]

In bazel, build-time and package code can be mixed freely. For CMake, this build-time code needs to be split into a separate package to prevent headaches and workarounds such as:

CompilerGym/compiler_gym/envs/llvm/make_specs.py

Lines 17 to 22 in f8a5117

	# TODO(github.com/facebookresearch/CompilerGym/issues/506): Avoids circular
	# dependency during specs.py generation, because it is imported from
	# compiler_gym.envs.llvm before being generated.
	sys.modules["compiler_gym.envs.llvm.is_making_specs"] = types.ModuleType(
	"compiler_gym.envs.llvm.is_making_specs"
	)

There are a couple of places that this needs updating, identified by TODO(github.com/facebookresearch/CompilerGym/issues/506).

This is related to #568.