sync.c : duplicate the target before modifying it

sync.c : duplicate the target before modifying it

It was probably a bad idea to modify the target directly in case of
repo/pkg syntax.

Duplicating it also allows us to keep the original target string, which
is more informative when printing errors.

Also remove a duplicated error message from libalpm, and improve the
message already returned to the frontend.

$ pacman -S foo/bar

before
error: repository 'foo' not found
error: 'bar': no such repository

after
error: 'foo/bar': could not find repository for target

Signed-off-by: Xavier Chantry <shiningxc@gmail.com>
Signed-off-by: Dan McGee <dan@archlinux.org>