Template include asset doesn't follow namespace lookup order

The namespace lookup order isn't followed when files are included with includeasset (includejs/includecss).

This causes an issue in events:
event foo
includejs foobar.js

Looks in the phpBB style template directory (e.g. styles/prosilver/template/) rather than the extension's directory (e.g. ext/foo/bar/styles/prosilver/template/).