Called by the processor when it encounters an xsl:include, xsl:import, or document() function.

  Called by the processor when it encounters
an xsl:include, xsl:import, or document() function.
Returns: A Source object, or null if the href cannot be resolved, and the processor should try to resolve the URI itself.
Parameters:
- href - An href attribute, which may be relative or absolute.
- base - The base URI against which the first argument will be made absolute if the absolute URI is required.
Throws:
- TransformerException - if an error occurs when trying to resolve the URI.