Ported AppSchemaValidator to JAXP to remove Xerces dependency

Justin,

this change ports AppSchemaValidator to JAXP to remove the explicit Xerces dependency. Note that xmlcodegen still fails to build (I commented it out for testing).

With noxmldeps GeoTools, there is a unit test failure in GeoServer (app-schema-test SecondaryNamespaceExceptionTest). Looks like a side effect of your dexercesification is that undefined namespace prefixes (null:) are now passed through the GML32OutputFormat XSLT rather that causing an ows:ExceptionReport. Not sure why. Also need to remove the xerces dependencies in the release module.

Kind regards,
Ben.