phabricator: move extension from contrib to hgext

It's well-enough tested now and widely enough used I think we should
ship it.

Differential Revision: https://phab.mercurial-scm.org/D4602

tests/test-check-help.t

@@ -26,4 +26,4 b' Check if ":hg:`help TOPIC`" is valid:'
   $ testrepohg files 'glob:{hgdemandimport,hgext,mercurial}/**/*.py' \
   > | sed 's|\\|/|g' \
   > | xargs $PYTHON "$TESTTMP/scanhelptopics.py" \
-  > | xargs -n1 hg help > /dev/null
+  > | xargs -n1 hg help --config extensions.phabricator= > /dev/null

tests/test-phabricator.t

@@ -1,7 +1,7 b''
 #require vcr
   $ cat >> $HGRCPATH <<EOF
   > [extensions]
-  > phabricator = $TESTDIR/../contrib/phabricator.py
+  > phabricator = 
   > EOF
   $ hg init repo
   $ cd repo