Restructure distribution units to allow deprecation of the 'files' attribute
Currently distribution units keep a "files" attribute that lists each file that is part of the distribution. This should be changed to allow the contents of the distribution unit to be overlaid on top of the repository without needing to list every file.
1) The (treeinfo/.treeinfo) file is renamed to "treeinfo" during the sync.
2) The existence of the treeinfo file is indicated on the unit object
3) During publish time the contents of the distribution are overlaid on the repository being generated.
4) The files attribute can be deprecated/removed