FindOpenSSL: Restore support for crypto-only result
Since commit 696339a9 we require both crypto and ssl
libraries to be present. This makes sense because OPENSSL_LIBRARIES
lists both and breaks when one is not found. However, prior to that
fix we supported finding only the crypto library and using it through
the imported target. Drop the requirement for ssl to restore support
for using crypto alone.
Later this module should be taught to support the COMPONENTS argument
of find_package.
Fixes: #16882 (closed)