Problem:
- Not obvious which _meta/ are generated and which should be edited
manually.
- The require guard (`error('Cannot require a meta file')`) is not
consistently present in all meta files.
Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.
(cherry picked from commit 65b40e69ac)
53038d2c38
