Make WebDAV build as a contrib project

Since the project has been moved to contrib it no longer builds, as it fails the validation step.
The class names / pom.xml needs to be adapted to build the project under its new location.

I propose to move the classes to the package org.xwiki.contrib.plugin.webdav. (In fact I already moved this locally and would only need to check it in.)

Anyone prefers a different package name?