add Fluent::Plugin::BareOutput to support v0.12 MultiOutput plugins

[tagomoris]

No description provided.

[tagomoris]

@sonots Please check this change

[sonots]

For the problem of ERROR2 of #973, I got another error for config-expander as

Error: test_emit(ConfigExpanderOutputTest): RuntimeError: BUG: use router.emit instead of Engine.emit

I also have one question: what does "bare" infer? How plugin developers choose a parent class, Output or BareOutput to create an output plugin?

[tagomoris]

It's intentional to prohibit Engine.emit... it will not be fixed so far.
BareOutput is very limited and skinny one, and it's not for almost plugin authors. it's just for meta plugins like forest, config-expander or something like that.

[sonots]

Okay, so compatibility layer for Engine.emit is not provided.
I understand that we have to tell about it to plugin developers.

Code itself looks good to me.

[tagomoris]

Once I thought about Engine.emit, but that is just under Fluent::, and no space to override for compatibilities. And that change is of v0.12 originally (we should have shown "deprecated" message in v0.12 versions).

[tagomoris]

I added some comment to tell not to use this for normal output plugins.