don't require to have local file in Manifest

This commit don't check the local file with the Manifest file, as this I consider this to be useless and even problematic (needing to run pybuild ... manifest). This doesn't change anything for the DIST file. Also, when creating a Manifest file, it will still write file dependancy, but won't use it. This will to keep compatibility with older version for a moment.

The fact that the Manifest are wrote with local file, and that local file are checked against the Manifest is controlled by two environment flag.