templater: look up mapping table through template engine
These functions are stub for symbol/resource separation. This series is
intended to address the following problems:
a) internal data may be exposed to user (
issue5699)
b) defaults['repo'] (a repository name) will conflict with mapping['repo']
(a repo object) in hgweb
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}