The main interface to eager loading is config.eager_load. The logic that
implies happens during the boot process.

With the introduction of Zeitwerk, application code is loaded in the
finisher as everything else, but in previous versions of Rails users
could eager load the application code regardless of config.eager_load.

Use cases:

   * Some gems like indexers need to have everything in memory and would
   be a bad user experience to ask users to conditionally set the eager
   load flag.

   * Some tests may need to have everything in memory and would be a bad
   experience to have the flag enabled globally in the test environment.

I personally feel that the contract between this method and the entire
eager loading process is ill-defined. I believe this method is
essentially internal. The purpose of this patch is simply to restore this
functionality emulating what it did before because rethinking the design
of this interface may need time.