R1:dd2b10b8f859

Guarantee repositories have unique local paths

Summary:
Ref T4039. Long ago these were more freely editable and there were some security concerns around creating a repository, then setting its local path to point somewhere it shouldn't.

Local paths are no longer editable so there's no real reason we need to provide a uniqueness guarantee anymore, but you could still make a mistake with `bin/repository move-paths` by accident, and it's a little cleaner to pull them out into their own column with a key.