Bug 1071012 - Extract Sphinx variables via AST reading; r=glandium

The in-tree Sphinx docs have been broken since bug 1041941 because
processing moz.build files outside their context doesn't work.
Specifically, templates aren't loaded (because this information usually
comes from a parent moz.build file). A new execution mode is needed.

I tried to implement a proper execution mode. However, I kept running
into walls. While we should strive for a proper execution mode, this can
be a follow-up, tracked in bug 1058359.

This patch implements extraction of Sphinx variables from ast walking.
It is extremely low-level and definitely a one-off. But it solves the
problem at hand: |mach build-docs| will work after this patch is
applied.

--HG--
extra : rebase_source : abd0a91a3efb24d3adfa19f4cd281ce5fd6d0915
extra : amend_source : c1b4f79224bab55e65a8c2b0f3103475281416c1