[FIXED HUDSON-7211] fix Hudson build on case-insensitive filesystems.

hudson/model/label/*.class conflicted with hudson/model/Label/*.jelly,
so renamed new package to hudson.model.labels


git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@33875 71c3de6d-444a-0410-be80-ed276b4c234a