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/nixexpr.hh

@@ -26,7 +26,7 @@ MakeError(RestrictedPathError, Error);
 
 struct Pos
 {
-    Symbol file;
+    std::string file;
     FileOrigin origin;
     uint32_t line;
     uint32_t column;
@@ -64,10 +64,10 @@ public:
         explicit Origin(uint32_t idx): idx(idx), file{}, origin{} {}
 
     public:
-        const Symbol file;
+        const std::string file;
         const FileOrigin origin;
 
-        Origin(Symbol file, FileOrigin origin): file(file), origin(origin) {}
+        Origin(std::string file, FileOrigin origin): file(std::move(file)), origin(origin) {}
     };
 
     struct Offset {