The web script framework has a built-in mechanism for calling other rest APIs. The root object is called "remote". The framework can connect to any system, even non-Alfresco systems, but, by default, it connects to the Alfresco repo tier.
So, you can do something like:
response = remote.call("/some/webscript");
And that will invoke a web script on the repo tier that has a URL of "/alfresco/service/some/webscript".