Support multiple source packages in yupdate.py

[Justin]

Using mintest as example.
source :
- https://github.com/minetest/minetest/archive/0.4.14.tar.gz : eebc2830d0e2f431f648691a0072fbb773d000c56deaf09ddc5e7416096cdf85
- https://github.com/minetest/minetest_game/archive/0.4.14.tar.gz : 203de4d41a60466126ab92ca85f726d88d0084f2e78393da239e7416cb847054

Have yupdate.py accept multiple variables ie yupdate.py https://github.com/minetest/minetest/archive/0.4.14.tar.gz https://github.com/minetest/minetest_game/archive/0.4.14.tar.gz

yupdate.py would then clear out the existing source urls/sha256s and replace them with those of the newly provided URLs.