This fixes a regression in db59e55fe7a0b67d3af868990468e7b8056afe42: prior
to that commit `mpremote` supported trailing slashes on the destination of
a normal (non-recursive) copy.
Add back support for that, with the semantics that a trailing slash
requires the destination to be an existing directory.
Also add a test for this.
Signed-off-by: Damien George <damien@micropython.org>