Cleanup Evalfile handling

This cleans up the EvalFile handling after the merge of #4915,
which has become a bit confusing on what it is actually doing.

closes https://github.com/official-stockfish/Stockfish/pull/4971

No functional change

src/evaluate.h

@@ -20,6 +20,7 @@
 #define EVALUATE_H_INCLUDED
 
 #include <string>
+#include <unordered_map>
 
 #include "types.h"
 
@@ -34,8 +35,6 @@ std::string trace(Position& pos);
 int   simple_eval(const Position& pos, Color c);
 Value evaluate(const Position& pos);
 
-extern std::string currentEvalFileName[2];
-
 // The default net name MUST follow the format nn-[SHA256 first 12 digits].nnue
 // for the build process (profile-build and fishtest) to work. Do not change the
 // name of the macro, as it is used in the Makefile.
@@ -44,11 +43,21 @@ extern std::string currentEvalFileName[2];
 
 namespace NNUE {
 
+enum NetSize : int;
+
 void init();
 void verify();
 
 }  // namespace NNUE
 
+struct EvalFile {
+    std::string option_name;
+    std::string default_name;
+    std::string selected_name;
+};
+
+extern std::unordered_map<NNUE::NetSize, EvalFile> EvalFiles;
+
 }  // namespace Eval
 
 }  // namespace Stockfish