Move /src to /subprojects

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

subprojects/libcmd/common-eval-args.hh

@@ -0,0 +1,73 @@
+#pragma once
+///@file
+
+#include "args.hh"
+#include "canon-path.hh"
+#include "common-args.hh"
+#include "search-path.hh"
+
+#include <filesystem>
+
+namespace nix {
+
+class Store;
+
+namespace fetchers { struct Settings; }
+
+class EvalState;
+struct EvalSettings;
+struct CompatibilitySettings;
+class Bindings;
+struct SourcePath;
+
+namespace flake { struct Settings; }
+
+/**
+ * @todo Get rid of global setttings variables
+ */
+extern fetchers::Settings fetchSettings;
+
+/**
+ * @todo Get rid of global setttings variables
+ */
+extern EvalSettings evalSettings;
+
+/**
+ * @todo Get rid of global setttings variables
+ */
+extern flake::Settings flakeSettings;
+
+/**
+ * Settings that control behaviors that have changed since Nix 2.3.
+ */
+extern CompatibilitySettings compatibilitySettings;
+
+struct MixEvalArgs : virtual Args, virtual MixRepair
+{
+    static constexpr auto category = "Common evaluation options";
+
+    MixEvalArgs();
+
+    Bindings * getAutoArgs(EvalState & state);
+
+    LookupPath lookupPath;
+
+    std::optional<std::string> evalStoreUrl;
+
+private:
+    struct AutoArgExpr { std::string expr; };
+    struct AutoArgString { std::string s; };
+    struct AutoArgFile { std::filesystem::path path; };
+    struct AutoArgStdin { };
+
+    using AutoArg = std::variant<AutoArgExpr, AutoArgString, AutoArgFile, AutoArgStdin>;
+
+    std::map<std::string, AutoArg> autoArgs;
+};
+
+/**
+ * @param baseDir Optional [base directory](https://nixos.org/manual/nix/unstable/glossary#gloss-base-directory)
+ */
+SourcePath lookupFileArg(EvalState & state, std::string_view s, const Path * baseDir = nullptr);
+
+}