submission_package: Move NCA reading code to its own function

This too, is completely separate behavior from what is in the
constructor, so we can move this to its own isolated function to keep
everything self-contained.

src/core/file_sys/submission_package.h

@@ -59,6 +59,8 @@ protected:
     bool ReplaceFileWithSubdirectory(VirtualFile file, VirtualDir dir) override;
 
 private:
+    void ReadNCAs(const std::vector<VirtualFile>& files);
+
     VirtualFile file;
 
     bool extracted;