This patch skips the resolution logic if the address is not local.
Today, the resolution logic handles that case transparently, and returns
the original email address, so this should be a no-op.
However, having an explicit early check makes the resolution logic more
robust, and will simplify future patches.
Note this also means that the `alias-resolve` hook is no longer run for
non-local aliases, which should also help simplify their implementation.