Enable local file downloads in the URLs

The possibility has been introduced in commit 055163004e68fef32374b0eef1...
But there was a condition making this impossible.
New type of <version> : no-url

Allow a user to define a package which does not have a URL.

My opinion on the subject is that the 'type' property should be implemented
at the <url> level in order to enable the presence of multiple, single or
no download URLs.
Considering this, my solution is a convenience one which prevents the code
for not diverging too from the official repository.