resourceutil: implement `is_resource()`
This will be needed when iterating resources.
Differential Revision: https://phab.mercurial-scm.org/D7773
#require vcr
$ cat >> $HGRCPATH <<EOF
> [extensions]
> phabricator =
> EOF
$ hg init repo
$ cd repo
$ cat >> .hg/hgrc <<EOF
> [phabricator]
> url = https://phab.mercurial-scm.org/
> callsign = HG
>
> [auth]
> hgphab.schemes = https
> hgphab.prefix = phab.mercurial-scm.org
> # When working on the extension and making phabricator interaction
> # changes, edit this to be a real phabricator token. When done, edit
> # it back. The VCR transcripts will be auto-sanitised to replace your real
> # token with this value.
> hgphab.phabtoken = cli-hahayouwish
> EOF
$ VCR="$TESTDIR/phabricator"