The SchemaResolver is a user-supplied class used for resolving references to
schema documents. It applies to references from one schema document to another
appearing in xs:import, xs:include, and xs:redefine; to
references from an instance document to a schema in xsi:schemaLocation and
xsi:noNamespaceSchemaLocation, to xsl:import-schema in XSLT, and to
the import schema declaration in XQuery.