XalanTransformer's EntityResolver fails to work on Posix platforms in case of entity file absence

A custom EntityResolver is not called when operating with XalanTransformer in situation when an ``entity'' file is absent.

The root of this problem goes from Xerces-C++ bug https://issues.apache.org/jira/browse/XERCESC-1807

But IMHO Xalan-C code is some kind miss-designed in one point (sorry for such brave suggestion): XSLT/XSLTProcessorEnvSupportDefault.cpp:145 (rev. 471584), when getting an URL from the ``urlString'' with the mechanism of local directory structure. The ``URISupport::getURLFromString'' function in its way uses Xerces-c's ``XMLPlatformUtils::getFullPath'' which operates uncommon in POSIX system and throws an exception if file does not exists. I consider that URL's construction is needed for the check some lines below, but such URL plays a role of a hash and constructing it on local directories hierarchy is not a good idea.

I don't actually know what is considered in all the cases of usage of ``URISupport::getURLFromString'' function, so I think it must be discussed how to fix this issue.

With all my respect to the developers.