Refactor: libcrmcommon: separate xpath-related functions into their own source code file

The following functions were moved from xml.c to a new xpath.c:

freeXpathObject()
getXpathResult()
dedupXpathResults()
xpath_search()
get_xpath_object_relative()
get_xpath_object()

At >6K lines, xml.c was getting a bit unwieldy to add new functions to.