Support the `url::` syntax that remotes handles

[DavisVaughan]

i.e.

Remotes:
    url::https://github.com/DavisVaughan/vctrs/archive/fix/safer-joint-order-proxy.tar.gz

I was about to launch a cloud_check() with this in my Remotes and I wanted to verify that it was set up correctly, so I ran pak::pak() and it failed because pak doesn't support this format, but remotes does