Use stricter check in debug/prelude

[st0012]

Your proposal

In prelude.rb, we use defined?(::DEBUGGER__) to check if debug/session has been loaded, which would have defined Kernel#debugger.

But it's also possible to have ::DEBUGGER__ defined and NOT having debug/session loaded, like require "debug/config", which we have in one of our gems.

It requires debug/config because we want to configure the debugger with these requirements:

• The configuration can happen before/after require "debug", as we're not in control of gems' loading order.
• We want to configure it in Ruby code.

So we have something like this:

require "debug/config"

sorbet_paths = Gem.loaded_specs["sorbet-runtime"].full_require_paths.freeze

# skip Sorbet related frames/backtrace when debugging
DEBUGGER__::CONFIG[:skip_path] = Array(DEBUGGER__::CONFIG[:skip_path]) + sorbet_paths

But that triggers prelude.rb's early return 😢

Therefore, I wonder if it's possible to make the condition a bit stricter, like if defined?(::DEBUGGER__::Session)?