core/core: Replace includes with forward declarations where applicable

The follow-up to e2457418da, which
replaces most of the includes in the core header with forward declarations.

This makes it so that if any of the headers the core header was
previously including change, then no one will need to rebuild the bulk
of the core, due to core.h being quite a prevalent inclusion.

This should make turnaround for changes much faster for developers.

src/yuzu/main.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <memory>
+#include <unordered_map>
 #include <QMainWindow>
 #include <QTimer>
 #include "core/core.h"
@@ -23,6 +24,10 @@ class ProfilerWidget;
 class WaitTreeWidget;
 enum class GameListOpenTarget;
 
+namespace FileSys {
+class VfsFilesystem;
+}
+
 namespace Tegra {
 class DebugContext;
 }
@@ -169,7 +174,7 @@ private:
     QString game_path;
 
     // FS
-    FileSys::VirtualFilesystem vfs;
+    std::shared_ptr<FileSys::VfsFilesystem> vfs;
 
     // Debugger panes
     ProfilerWidget* profilerWidget;