packaging: move DOWNLOADS dict to hgpackaging.downloads
We'll want to keep state in sync between multiple packaging tools.
It makes sense to share a central data structure defining downloads.
We also change the function to return the downloads entry so
callers don't have to access the global DOWNLOADS in the new
location.
Differential Revision: https://phab.mercurial-scm.org/D6085
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}