Allow xsd shema file as a jaxb binding file to pass into JAXB schmeCompiler

This can pass some missed import schema files into jaxb with -b flag to generate types classes from some MS wsdl files. Then we can use wsdl2java to generate the follow wsdl with "wsdl2java -b http://www.w3.org/2001/XMLSchema.xsd following.wsdl"

<wsdl:types>
<schema targetNamespace="http://cxf.apache.org/w2j/hello_world_soap_http/types" xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:x1="http://cxf.apache.org/w2j/hello_world_soap_http/types" elementFormDefault="qualified">
<element name="sayHi">
<complexType/>
</element>
<element name="sayHiResponse">
<complexType>
<sequence>
<element ref="xsd:schema"/>
</sequence>
</complexType>
</element>
</schema>
</wsdl:types>