ENGINE modules aren't special, so call them MODULES

The only thing that makes an ENGINE module special is its entry
points.  Other than that, it's a normal dynamically loadable module,
nothing special about it.  This change has us stop pretending anything
else.

We retain using ENGINE as a term for installation, because it's
related to a specific installation directory, and we therefore also
mark ENGINE modules specifically as such with an attribute in the
build.info files.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/8147)