libstore/derivation-goal: avoid double-parsing of JSON messages

To avoid that JSON messages are parsed twice in case of
remote builds with `ssh-ng://`, I split up the original
`handleJSONLogMessage` into three parts:

* `parseJSONMessage(const std::string&)` checks if it's a message in the
  form of `@nix {...}` and tries to parse it (and prints an error if the
  parsing fails).
* `handleJSONLogMessage(nlohmann::json&, ...)` reads the fields from the
  message and passes them to the logger.
* `handleJSONLogMessage(const std::string&, ...)` behaves as before, but
  uses the two functions mentioned above as implementation.

In case of `ssh-ng://`-logs the first two methods are invoked manually.

src/libutil/logging.hh

@@ -4,6 +4,8 @@
 #include "error.hh"
 #include "config.hh"
 
+#include <nlohmann/json.hpp>
+
 namespace nix {
 
 typedef enum {
@@ -166,6 +168,12 @@ Logger * makeSimpleLogger(bool printBuildLogs = true);
 
 Logger * makeJSONLogger(Logger & prevLogger);
 
+std::optional<nlohmann::json> parseJSONMessage(const std::string & msg);
+
+bool handleJSONLogMessage(nlohmann::json & json,
+    const Activity & act, std::map<ActivityId, Activity> & activities,
+    bool trusted);
+
 bool handleJSONLogMessage(const std::string & msg,
     const Activity & act, std::map<ActivityId, Activity> & activities,
     bool trusted);