praefect: Handle replica paths in 'track-repository' and 'track-repositories' subcommands

Historically the Praefect tracking subcommands have not distinguished between the relative path used by clients and the replica path used internally by Praefect. Now that replica paths are in common usage we need to make it clear which path the user is specifying.

Rename the -repository flag to -relative-path, which matches the names used in our documentation, and then update the track-repository and track-repositories subcommands to use it.

Closes #5402 (closed)