Make the logger customisable

Add a new `--log-format` cli argument to change the format of the logs.
The possible values are
- raw (the default one for old-style commands)
- bar (the default one for new-style commands)
- bar-with-logs (equivalent to `--print-build-logs`)
- internal-json (the internal machine-readable json format)

src/libmain/loggers.cc

@@ -0,0 +1,47 @@
+#include "loggers.hh"
+#include "../nix/progress-bar.hh"
+
+namespace nix {
+
+LogFormat defaultLogFormat = LogFormat::raw;
+
+LogFormat parseLogFormat(const string &logFormatStr) {
+    if (logFormatStr == "raw")
+        return LogFormat::raw;
+    else if (logFormatStr == "internal-json")
+        return LogFormat::internalJson;
+    else if (logFormatStr == "bar")
+        return LogFormat::bar;
+    else if (logFormatStr == "bar-with-logs")
+        return LogFormat::barWithLogs;
+    throw Error(format("option 'log-format' has an invalid value '%s'") %
+                logFormatStr);
+}
+
+Logger *makeDefaultLogger() {
+    switch (defaultLogFormat) {
+    case LogFormat::raw:
+        return makeSimpleLogger();
+    case LogFormat::internalJson:
+        return makeJSONLogger(*makeSimpleLogger());
+    case LogFormat::bar:
+        return makeProgressBar();
+    case LogFormat::barWithLogs:
+        return makeProgressBar(true);
+    }
+}
+
+void setLogFormat(const string &logFormatStr) {
+    setLogFormat(parseLogFormat(logFormatStr));
+}
+
+void setLogFormat(const LogFormat &logFormat) {
+    defaultLogFormat = logFormat;
+    createDefaultLogger();
+}
+
+void createDefaultLogger() {
+    logger = makeDefaultLogger();
+}
+
+}