vfs: Use existing type aliases consistently

Makes use of the VirtualDir and VirtualFile aliases across the board
instead of having a few isolated places that don't use it.

src/core/file_sys/vfs_static.h

@@ -31,7 +31,7 @@ public:
         return true;
     }
 
-    std::shared_ptr<VfsDirectory> GetContainingDirectory() const override {
+    VirtualDir GetContainingDirectory() const override {
         return parent;
     }