move headers around

this moves the headers for GL/VK to their corresponding subdirs (src/mesa should probably be src/gl now but that's a separate project). with that done, CI rules can be updated to avoid unnecessarily triggering jobs.

fixes #8995

ci https://gitlab.freedesktop.org/zmike/mesa/-/pipelines/928566

future work: move CL headers to rusticl once clover is deleted, maybe move d3d headers to the nine dir?