find_cached_package: avoid adding duplicates

If PKGDEST is set when makepkg was run, the package will be present in
find_cached_package's search path by default, causing an error.

This also fixes a display bug which causes no output to be shown when
multiple packages are found.

Fixes FS#37626.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>