module-xml: reused namespace prefixes in external schemas cause an exception

[davidnich]

refs #3194

ex:

WSDL-NAMESPACE-ERROR: imported schema uses prefix "ns0" to refer to "http://www.infinica.com/", however this prefix was previously defined with "http://www.infinica.com/datastore"
call stack:
  7: Namespaces::merge() (/export/home2/dnichols/src/qore/git/module-xml/qlib/WSDL.qm:4864 (Qore user code))
  6: WebService::parseTypes() (/export/home2/dnichols/src/qore/git/module-xml/qlib/WSDL.qm:4871 (Qore user code))
  5: WebService::parseTypes() (/export/home2/dnichols/src/qore/git/module-xml/qlib/WSDL.qm:4871 (Qore user code))
  4: WebService::parseTypes() (/export/home2/dnichols/src/qore/git/module-xml/qlib/WSDL.qm:5122 (Qore user code))
  3: WebService::doInit() (/export/home2/dnichols/src/qore/git/module-xml/qlib/WSDL.qm:4624 (Qore user code))
  2: WebService::constructor() (/export/home2/dnichols/src/qore/git/module-xml/qlib/SoapClient.qm:253 (Qore user code))
  1: SoapClient::constructor() (<command-line>:1 (Qore user code))