package.sh: When downloading files, if file exists, check that it has the right size
package.sh: When downloading files, if file exists, check that it has the right size

If a file was not downloaded correctly, this caused an issue where it would
never try to redownload it. This should fix it (not tested properly though)

alephobjects/Cura,alephobjects/Cura,alephobjects/Cura