Don't treat .d (depend) files separately from object files

.d (.MMS in the VMS world) files with just dependencies are built from
exactly the same conditions as the object files.  Therefore, the rules
for them can be built at the same time as the rules for the
corresponding object files.

This removes the requirement for a src2dep function in the build file
templates, and for common.tmpl to call it.  In the end, the existence
of depend files is entirely up to the build file.

Reviewed-by: Rich Salz <rsalz@openssl.org>