Update Zeitwerk setup

[fxn]

What? Why?

There are three proposed changes.

The main motivation for this patch is that thanks to fxn/zeitwerk#282 and fxn/zeitwerk#284 I have learned __dir__ is not a good path prefix for setting loaders up, because it is a real path, while __FILE__ is not. This detail shows up when using symlinks, as it is the case in the project those tickets were about. The proposed code uses File.dirname(__FILE__) instead, for cosistency in the involved paths.

Gruf probably copied a snippet from the README that unrolled what for_gem does and used __dir__, that snippet has been recently updated (fxn/zeitwerk@7daca61). However, that snippet is only there for illustration purposes, it is more simple and idiomatic to setup the loader with for_gem. So we simplify things a bit with for_gem.

Finally, since I was on it, Zeitwerk is a top-level constant, there is no reason to use a leading :: in the top-level of the script.

How was it tested?

Using demo apps locally, both auto and eager loading.

[splittingred]

Hi @fxn - can you rebase from latest here?

[fxn]

@splittingred done!

[splittingred]

Thanks!