Move /src to /subprojects

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

subprojects/libmain/common-args.hh

@@ -0,0 +1,70 @@
+#pragma once
+///@file
+
+#include "args.hh"
+#include "repair-flag.hh"
+
+namespace nix {
+
+//static constexpr auto commonArgsCategory = "Miscellaneous common options";
+static constexpr auto loggingCategory = "Logging-related options";
+static constexpr auto miscCategory = "Miscellaneous global options";
+
+class MixCommonArgs : public virtual Args
+{
+    void initialFlagsProcessed() override;
+public:
+    std::string programName;
+    MixCommonArgs(const std::string & programName);
+protected:
+    virtual void pluginsInited() {}
+};
+
+struct MixDryRun : virtual Args
+{
+    bool dryRun = false;
+
+    MixDryRun()
+    {
+        addFlag({
+            .longName = "dry-run",
+            .description = "Show what this command would do without doing it.",
+            //.category = commonArgsCategory,
+            .handler = {&dryRun, true},
+        });
+    }
+};
+
+struct MixJSON : virtual Args
+{
+    bool json = false;
+
+    MixJSON()
+    {
+        addFlag({
+            .longName = "json",
+            .description = "Produce output in JSON format, suitable for consumption by another program.",
+            //.category = commonArgsCategory,
+            .handler = {&json, true},
+        });
+    }
+};
+
+struct MixRepair : virtual Args
+{
+    RepairFlag repair = NoRepair;
+
+    MixRepair()
+    {
+        addFlag({
+            .longName = "repair",
+            .description =
+                "During evaluation, rewrite missing or corrupted files in the Nix store. "
+                "During building, rebuild missing or corrupted store paths.",
+            .category = miscCategory,
+            .handler = {&repair, Repair},
+        });
+    }
+};
+
+}