Remove std::string alias (for real this time)

Also use std::string_view in a few more places.

src/libexpr/common-eval-args.hh

@@ -22,6 +22,6 @@ private:
     std::map<std::string, std::string> autoArgs;
 };
 
-Path lookupFileArg(EvalState & state, string s);
+Path lookupFileArg(EvalState & state, std::string_view s);
 
 }