clonebundles: move the manifest reading in a dedicated function
We are about to make the logic more advanced to help hosting solution, so we
need to centralize it first.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}