[RIF] xml serializations for Import

In validating the Core test cases against the PRD schema there's one
issue, which is that Core and PRD specify different serializations for
the arguments of Import (location and profile).

Here's an import directive serialized according to Core:

    <directive>
       <Import>
          <location>
               <Const
type="&xs;anyURI">http;//www.w3.org/2005/rules/test/Core/IRI_from_RDF_Literal/IRI_from_RDF_Literal-import001.nt</Const>
          </location>
          <profile>
               <Const
type="&xs;anyURI">http;//www.w3.org/2007/rif-import-profile#Simple</Const>
          </profile>
       </Import>
    </directive>

This isn't accepted according to the PRD schema which has location and
profile as simple types.  PRD specifies the serialization as:
    <directive>
       <Import>
          <location>
               http;//www.w3.org/2005/rules/test/Core/IRI_from_RDF_Literal/IRI_from_RDF_Literal-import001.nt
          </location>
          <profile>
               http;//www.w3.org/2007/rif-import-profile#Simple
          </profile>
       </Import>
    </directive>

Stella