Move /src to /subprojects

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

subprojects/libexpr/attr-path.hh

@@ -0,0 +1,27 @@
+#pragma once
+///@file
+
+#include "eval.hh"
+
+#include <string>
+#include <map>
+
+namespace nix {
+
+MakeError(AttrPathNotFound, Error);
+MakeError(NoPositionInfo, Error);
+
+std::pair<Value *, PosIdx> findAlongAttrPath(
+    EvalState & state,
+    const std::string & attrPath,
+    Bindings & autoArgs,
+    Value & vIn);
+
+/**
+ * Heuristic to find the filename and lineno or a nix value.
+ */
+std::pair<SourcePath, uint32_t> findPackageFilename(EvalState & state, Value & v, std::string what);
+
+std::vector<Symbol> parseAttrPath(EvalState & state, std::string_view s);
+
+}