package: ensure do_split_packages doesn't return duplicates

package: ensure do_split_packages doesn't return duplicates

do_split_package() constructs a list of packages that were created as it
iterates through the files, so if multiple files go into the same package then
the package will be repeated in the output.

Solve this by using a set() to store the created packages so that duplicates are
ignored.

Signed-off-by: Ross Burton <ross.burton@intel.com>