Filter files we don't want to preload

Since we built the list of files to be preloaded from opcache statistics, a lot of files we don't want to preload are also included. Typically, we shouldn't preload any files that have a side-effect.

We should only be preloading files that only have a class definition. One way we can avoid other files is by filtering out files that have words like `functions` or `bootstrap` in its name.