Allow to check the absence of some properties in XML files of a XAR at build time

My usecase is to be able to detect automatically at build time that a XML file does not contain a specific xobject when building a XAR module. To be more specific, we got a regression recently (XWIKI-18975) whose root cause was related to some xobjects wrongly committed on the repo.

Now we could make this check a bit more generic, than only checking for xobjects absence, and allow to add a configuration to check if a xpath is present or missing in a file, depending on what we want to check.