Implement safer filesystem level calls

[e-dard]

In a number of places we are using os.Rename for example. This command (on *nix) will happily overwrite files that might already exist at dest.

We should provide a higher level API that checks if files exist and allows us to assert that our invariants are true, e.g., that we're never renaming a file over another file if we don't intend to.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[e-dard]

This will be closed via #14280