all_urls: add urls[0] for versions (#16435)
all_urls: add urls[0] for versions (#16435)

This adds the `url` alternative `urls` to `package.all_urls`. With
this addition, one can find again new versions with
`spack versions <package>` for packages that are populated with
from mixin mirror `urls`.

Example: `util-macros` from x.org mixin.
LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack