Add shell completion for --override-flake

Requires moving the MixEvalArgs class from libexpr to libcmd because
that's where completeFlakeRef is.

src/libcmd/common-eval-args.hh

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "args.hh"
+
+namespace nix {
+
+class Store;
+class EvalState;
+class Bindings;
+
+struct MixEvalArgs : virtual Args
+{
+    MixEvalArgs();
+
+    Bindings * getAutoArgs(EvalState & state);
+
+    Strings searchPath;
+
+    std::optional<std::string> evalStoreUrl;
+
+private:
+    std::map<std::string, std::string> autoArgs;
+};
+
+Path lookupFileArg(EvalState & state, std::string_view s);
+
+}