Replaced XMLSrc with StructFile

XMLSrc and StructFile were very different implementations of basically
the same thing.  StructFile has gotten lots and lots of improvements,
while XMLSrc, with its wonky, unintuitive interface, has floundered.
This replaces XMLSrc with StructFile (nearly) everywhere, and rebases
everything that inherited from XMLSrc with StructFile.

XMLSrc lives on somewhat in the Pkgmgr plugin's PNode and PkgSrc
objects, where originally inherited from the XMLSrc and INode objects
but are now the only implementation of that older interface.  Pkgmgr
was left as-is because a) it's a little-used plugin; b) it's in need
of a rewrite anyway; and c) it has deep, deep hooks into the guts of
XMLSrc and INode, and rewriting it with StructFile was deeply
nontrivial.
testsuite: updated Statistics tests with new library location