Bug 1109302 - make moz.build *SOURCES variables emit proper objects; r=gps

Having SOURCES and its close relatives go through VariablePassthru
objects clutters the handling of VariablePassthru in build backends and
makes it less obvious how to handle things that actually get compiled.

Therefore, this patch introduces four new moz.build objects
corresponding to the major variants of SOURCES.  It looks like a large
patch, but there's an ample amount of new tests included, which accounts
for about half of the changes.