Build file templates: rework how general dependencies are computed

Build file templates: rework how general dependencies are computed

For some types of targets, we pretty much know what kinds of files all
the dependencies are.  For some, however, we can't assume anything,
and are faced with dependencies in platform agnostic form.  We need to
find those in diverse places in %unified_info, and deduce from there
how they should be converted to a platform specific form.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15436)