Use a different hack in the ${DFILE} rule to keep indirect dependencies

Use a different hack in the ${DFILE} rule to keep indirect dependencies
(mostly for includes) separate from direct dependencies (so that ${.ALLSRC}
can be used to find full paths to the sources for the direct dependencies
only).  The `::' hack just forgot the indirect dependendencies.  This
broke building doc/usd/13.viref with `make -jN' - the index got corrupted
by being built twice concurrently.

Cleaned up the ${DFILE} rule.  There was a .else clause with dead code in
it following a .else clause (make accepts this bad syntax).  ${.ALLSRC}
now works in the USE_SOELIMPP case.  Some client Makefiles no longer need
the SRCDIR=${.OBJDIR} hack.