don't use Symbol in Pos to represent a path

PosTable deduplicates origin information, so using symbols for paths is no
longer necessary. moving away from path Symbols also reduces the usage of
symbols for things that are not keys in attribute sets, which will become
important in the future when we turn symbols into indices as well.

src/libexpr/eval.hh

@@ -75,6 +75,8 @@ public:
     SymbolTable symbols;
     PosTable positions;
 
+    static inline std::string derivationNixPath = "//builtin/derivation.nix";
+
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
         sFile, sLine, sColumn, sFunctor, sToString,