Use a struct not std::pair for SearchPathElem

I got very confused trying to keep all the `first` and `second` straight
reading the code, *especially* as there is also another `(boolean,
string)` pair type also being used.

Named fields is much better.

There are other cleanups that we can do (for example, the existing
TODO), but we can do them later. Doing them now would just make this
harder to review.

src/libexpr/eval.hh

@@ -65,8 +65,12 @@ std::string printValue(const EvalState & state, const Value & v);
 std::ostream & operator << (std::ostream & os, const ValueType t);
 
 
-// FIXME: maybe change this to an std::variant<SourcePath, URL>.
-typedef std::pair<std::string, std::string> SearchPathElem;
+struct SearchPathElem
+{
+    std::string prefix;
+    // FIXME: maybe change this to an std::variant<SourcePath, URL>.
+    std::string path;
+};
 typedef std::list<SearchPathElem> SearchPath;