FindOpenSSL: Restore support for crypto-only result

Since commit v3.8.0-rc1~132^2 (FindOpenSSL: Check that both CRYPTO and
SSL libraries are present, 2017-01-03) 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

Modules/FindOpenSSL.cmake

@@ -375,7 +375,7 @@ set(OPENSSL_LIBRARIES ${OPENSSL_SSL_LIBRARY} ${OPENSSL_CRYPTO_LIBRARY} )
 if (OPENSSL_VERSION)
   find_package_handle_standard_args(OpenSSL
     REQUIRED_VARS
-      OPENSSL_SSL_LIBRARY
+      #OPENSSL_SSL_LIBRARY # FIXME: require based on a component request?
       OPENSSL_CRYPTO_LIBRARY
       OPENSSL_INCLUDE_DIR
     VERSION_VAR
@@ -385,7 +385,7 @@ if (OPENSSL_VERSION)
   )
 else ()
   find_package_handle_standard_args(OpenSSL "Could NOT find OpenSSL, try to set the path to OpenSSL root folder in the system variable OPENSSL_ROOT_DIR"
-    OPENSSL_SSL_LIBRARY
+    #OPENSSL_SSL_LIBRARY # FIXME: require based on a component request?
     OPENSSL_CRYPTO_LIBRARY
     OPENSSL_INCLUDE_DIR
   )