Move /src to /subprojects

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

subprojects/libexpr/paths.cc

@@ -0,0 +1,15 @@
+#include "eval.hh"
+
+namespace nix {
+
+SourcePath EvalState::rootPath(CanonPath path)
+{
+    return {rootFS, std::move(path)};
+}
+
+SourcePath EvalState::rootPath(PathView path)
+{
+    return {rootFS, CanonPath(absPath(path))};
+}
+
+}