re2: fix reporting of availability in `hg debuginstall`
We add and use an official API to check is re2 is available. This prevent the
bug previously in place were debuginstall was considering confusing `None`
(i.e. non-initialized) and `False` (i.e. unavailable).
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}