debian: Remove the extrinsic version from release names

Use only the intrinsic version (eg. 1.0.0) instead of the extrinsic version
(eg. stretch/contrib/1.0.0).
Releases should only contain data from DSC, not external 'pointers' to them.

Additionally, having extrinsic data in releases means the same
dsc-sha256 extid can point to different releases, which meant the loader
may reuse a release mentioning a specific suite as a release in a
different suite.
With this commit, this won't be a problem anymore, as releases won't
mention the suite at all, so suites can safely share extids.