Factor out nix-defexpr path computation

Avoid duplicated code, and also avoid "on the fly" path construction
(which makes it harder to keep track of which paths we use).

The factored out code doesn't create the Nix state dir anymore, but this
is fine because other in nix-env and nix-channel does:

- nix-channel: Line 158 in this commit

- nix-env: Line 1407 in this commit

src/libexpr/eval-settings.hh

@@ -95,4 +95,9 @@ struct EvalSettings : Config
 
 extern EvalSettings evalSettings;
 
+/**
+ * Conventionally part of the default nix path in impure mode.
+ */
+Path getNixDefExpr();
+
 }