Support @library instead of <library> notation in SOURCE

[hostilefork]

The source command can look up the URL of a script which is in the shorthands list.

This used to use tags, e.g. source <chess>

Being strings that can hold arbitrary paths, tags were retaken for an alternative path semantics (relative to script directory, vs. to current directory)

So the shorthands now use @. This should be changed to support source @chess