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.

lib/spack/spack/package.py

@@ -2011,6 +2011,10 @@ def all_urls(self):
         if hasattr(self, 'url') and self.url:
             urls.append(self.url)
 
+        # fetch from first entry in urls to save time
+        if hasattr(self, 'urls') and self.urls:
+            urls.append(self.urls[0])
+
         for args in self.versions.values():
             if 'url' in args:
                 urls.append(args['url'])