handle PURLs just before identifying artifact

this fixes handleSchemaImports which is designed to run without a
dependency on the PURL generator, as it is a vital function for bringing
up the permanent repository
Fix test that started to break now that unmanaged artifacts throw
exceptions
fix test that now relies on exceptions rather than an empty set
Move out essential code to OntologyUtils to easily unit test it

Difficult to test this method using PoddArtifactManager as it needs to
load an artifact initially, which requires identification of the schema
imports, before being able to do anything with getSchemaImports, which
is only valuable after the load is complete