Cannot generate migration on /core

[kennyadsl]

It's not currently possible to run rails commands on solidus (sub-)gems, due to a load error with Spree::Event::Subscriber not being loaded in time.

Solidus Version:
master

To Reproduce

> cd solidus
> cd core
> bundle exec rails generate migration add_fullname_to_spree_addresses fullname:string
Traceback (most recent call last):
	20: from script/rails:10:in `<main>'
	19: from script/rails:10:in `require'
	18: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/engine/commands.rb:9:in `<top (required)>'
	17: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/engine/commands.rb:9:in `require'
	16: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/commands.rb:18:in `<top (required)>'
	15: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/command.rb:46:in `invoke'
	14: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/command/base.rb:65:in `perform'
	13: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/thor-0.20.3/lib/thor.rb:387:in `dispatch'
	12: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/thor-0.20.3/lib/thor/invocation.rb:126:in `invoke_command'
	11: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/thor-0.20.3/lib/thor/command.rb:27:in `run'
	10: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/commands/generate/generate_command.rb:21:in `perform'
	 9: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/command/actions.rb:14:in `require_application_and_environment!'
	 8: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/command/actions.rb:19:in `require_application!'
	 7: from /Users/kennyadsl/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/railties-5.2.3/lib/rails/command/actions.rb:19:in `require'
	 6: from /Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/core/engine.rb:4:in `<top (required)>'
	 5: from /Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/core/engine.rb:4:in `require'
	 4: from /Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/event/processors/mailer_processor.rb:3:in `<top (required)>'
	 3: from /Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/event/processors/mailer_processor.rb:4:in `<module:Spree>'
	 2: from /Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/event/processors/mailer_processor.rb:5:in `<module:Event>'
	 1: from /Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/event/processors/mailer_processor.rb:6:in `<module:Processors>'
/Users/kennyadsl/Code/nebulab/solidus/core/lib/spree/event/processors/mailer_processor.rb:7:in `<module:MailerProcessor>': uninitialized constant Spree::Event::Subscriber (NameError)

Additional context

I suspect this could be fixed by #3232, cc @elia.
Tried on that branch as well, it still happens.