Move /src to /subprojects

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

subprojects/libcmd/repl.hh

@@ -0,0 +1,45 @@
+#pragma once
+///@file
+
+#include "eval.hh"
+
+namespace nix {
+
+struct AbstractNixRepl
+{
+    ref<EvalState> state;
+    Bindings * autoArgs;
+
+    AbstractNixRepl(ref<EvalState> state)
+        : state(state)
+    { }
+
+    virtual ~AbstractNixRepl()
+    { }
+
+    typedef std::vector<std::pair<Value*,std::string>> AnnotatedValues;
+
+    using RunNix = void(Path program, const Strings & args, const std::optional<std::string> & input);
+
+    /**
+     * @param runNix Function to run the nix CLI to support various
+     * `:<something>` commands. Optional; if not provided,
+     * everything else will still work fine, but those commands won't.
+     */
+    static std::unique_ptr<AbstractNixRepl> create(
+        const LookupPath & lookupPath,
+        nix::ref<Store> store,
+        ref<EvalState> state,
+        std::function<AnnotatedValues()> getValues,
+        RunNix * runNix = nullptr);
+
+    static ReplExitStatus runSimple(
+        ref<EvalState> evalState,
+        const ValMap & extraEnv);
+
+    virtual void initEnv() = 0;
+
+    virtual ReplExitStatus mainLoop() = 0;
+};
+
+}