Move c/include/cccl/*.h files to c/include/cccl/c/*.h

[rwgk]

Description

Closes: #2274

Piggy-backed: Change #warning to #error (to improve the user experience).

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[github-actions]

🟩 CI finished in 14m 44s: Pass: 100%/1 | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s

• 🟩 pycuda: Pass: 100%/1 | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s

  🟩 cpu
    🟩 amd64              Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 ctk
    🟩 12.5               Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 cudacxx
    🟩 nvcc12.5           Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 cudacxx_family
    🟩 nvcc               Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 cxx
    🟩 GCC13              Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 cxx_family
    🟩 GCC                Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 gpu
    🟩 v100               Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  🟩 jobs
    🟩 Test               Pass: 100%/1   | Total: 14m 44s | Avg: 14m 44s | Max: 14m 44s
  

👃 Inspect Changes

Modifications in project?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
	CUDA Experimental
	pycuda
+/-	CUDA C Core Library

Modifications in project or dependencies?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
	CUDA Experimental
+/-	pycuda
+/-	CUDA C Core Library

🏃‍ Runner counts (total jobs: 1)

#	Runner
1	linux-amd64-gpu-v100-latest-1

[alliepiper]

FYI: Merging in main to use new, smaller PR matrix.

[github-actions]

🟩 CI finished in 15m 13s: Pass: 100%/1 | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s

• 🟩 pycuda: Pass: 100%/1 | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s

  🟩 cpu
    🟩 amd64              Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 ctk
    🟩 12.5               Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 cudacxx
    🟩 nvcc12.5           Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 cudacxx_family
    🟩 nvcc               Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 cxx
    🟩 GCC13              Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 cxx_family
    🟩 GCC                Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 gpu
    🟩 v100               Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  🟩 jobs
    🟩 Test               Pass: 100%/1   | Total: 15m 13s | Avg: 15m 13s | Max: 15m 13s
  

👃 Inspect Changes

Modifications in project?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
	CUDA Experimental
	pycuda
+/-	CUDA C Core Library

Modifications in project or dependencies?

	Project
	CCCL Infrastructure
	libcu++
	CUB
	Thrust
	CUDA Experimental
+/-	pycuda
+/-	CUDA C Core Library

🏃‍ Runner counts (total jobs: 1)

#	Runner
1	linux-amd64-gpu-v100-latest-1