Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libmain/loggers.cc

@@ -0,0 +1,56 @@
+#include "loggers.hh"
+#include "environment-variables.hh"
+#include "progress-bar.hh"
+
+namespace nix {
+
+LogFormat defaultLogFormat = LogFormat::raw;
+
+LogFormat parseLogFormat(const std::string & logFormatStr) {
+    if (logFormatStr == "raw" || getEnv("NIX_GET_COMPLETIONS"))
+        return LogFormat::raw;
+    else if (logFormatStr == "raw-with-logs")
+        return LogFormat::rawWithLogs;
+    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("option 'log-format' has an invalid value '%s'", logFormatStr);
+}
+
+Logger * makeDefaultLogger() {
+    switch (defaultLogFormat) {
+    case LogFormat::raw:
+        return makeSimpleLogger(false);
+    case LogFormat::rawWithLogs:
+        return makeSimpleLogger(true);
+    case LogFormat::internalJSON:
+        return makeJSONLogger(*makeSimpleLogger(true));
+    case LogFormat::bar:
+        return makeProgressBar();
+    case LogFormat::barWithLogs: {
+        auto logger = makeProgressBar();
+        logger->setPrintBuildLogs(true);
+        return logger;
+    }
+    default:
+        unreachable();
+    }
+}
+
+void setLogFormat(const std::string & logFormatStr) {
+    setLogFormat(parseLogFormat(logFormatStr));
+}
+
+void setLogFormat(const LogFormat & logFormat) {
+    defaultLogFormat = logFormat;
+    createDefaultLogger();
+}
+
+void createDefaultLogger() {
+    logger = makeDefaultLogger();
+}
+
+}