Unix Makefile generator: separate "simple" shared libraries from import libraries

Unix Makefile generator: separate "simple" shared libraries from import libraries

For Unix like environments, we may have so called "simple" shared
library names (libfoo.so as opposed to libfoo.so.1.2), or we may have
"import" library names associated with a DLL (libfoo.dll.a for
libfoo.dll on Mingw and derivatives).

So far, "import" library names were treated the same as "simple"
shared library names, as some kind of normalization for the Unix way
of doing things.

We now shift to treat them separately, to make it clearer what is
what.

Fixes #13414, incidently

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13875)