feat: validate filenames in meet directories

In order to standardise the structure of the repository a little more and provide better errors when meets fail to process, we can validate that the meet directories contain a specific set of filenames. This will catch errors such as Entries.csv or other issues.

This is relatively relaxed for the time being, largely just picking the set of files that already exist until the error count is low enough to manually fix the rest.

This change:

• Adds some validation to the checker to ensure meet directories contain sensibly named files