fix: remove ModuleToolbox from public API

Remove ModuleToolbox from public API.
Also fixes an issue where an internal include was added in module.hh which is a public header.

I found a plugin that uses the ModuleToolbox. Here is an idea to remove its dependency to ModuleToolbox:

-                       sip_header_insert(msg, sip, (sip_header_t*)sip_unknown_format(msg_home(msg), "X-Target-Uris: %s", oss.str().c_str()));
+                       if(!sip_header_insert(msg, sip, (sip_header_t*)sip_unknown_format(msg_home(msg), "X-Target-Uris: %s", oss.str().c_str()))) {
+                               SLOGE << "an error has occurred while inserting X-Target-Uris header";
+                       }
+
                        SLOGD << "Generated header is X-Target-Uris: " << oss.str().c_str();
 
                        // Remove the preroute
                        char* params = su_strdup(msg_home(msg), sipurl->url_params);
                        sipurl->url_params = url_strip_param_string(params, "preroute");
-
-                       // Ensure we correctly added the X-Target-Uris header
-                       sip_unknown_t* h = ModuleToolbox::getCustomHeaderByName(ev->getSip(), "X-Target-Uris");
-                       if (!h) {
-                               SLOGE << "ERROR: X-Target-Uris header not found !";
-                       }
                }
        }