Add support for scriptlet aliases

[cipriancraciun]

Add support for aliasing a given scriptlet under multiple labels.

This can be used in multiple use-cases:

• if one renames a scriptlet's label, one has to replace it in all used places, thus an alias could provide backward compatibility;
• one could provide libraries of scriptlets with a "namespace" prefix (for example a random token, or a globally unique token), and the use could re-export those under simpler aliases;
• one could use a shorter alias as a shortcut;