build: remove stale .ipk files if package dir changes

build: remove stale .ipk files if package dir changes

If a package nonshared status is changed, a stale .ipk file might still
be present in the old package directory. Remove the .ipk file from all
package directories when building a new one (or explicitly running
clean)

Signed-off-by: Felix Fietkau <nbd@nbd.name>