Make `filesystem.partition` idempotent

This commit makes the `partition()` operation idempotent by checking
whether the requested device is already partitioned and no-op'ing if
so.

Right now this works via a hack (assume that partitioning "/dev/FOO"
produces "/dev/FOO1"), but the alternatives would take some work/time
that we don't have right now.

In the future there may be cleaner alternatives.