Remove dependency on `ActiveSupport::Concern`

[cupakromer]

We want to move away from using ActiveSupport::Concern. Where we use it there isn't a large benefit to warrant pulling the dependency. Pulling this out will help move toward providing better isolated testing and cherry-picking of rspec-rails.

We should move to either standard Ruby modules or using shared example groups. The first step is probably the standard Ruby modules. Once a decision on rspec/rspec-core#1749 has been made we can decide how best to utilize shared example groups.

[myronmarston]

Honestly, I think we might be better off moving to ruby modules and ignoring rspec/rspec-core#1749 here. (Although, that PR does affect module inclusion as well). Shared example groups have some nice extra features (they support arguments, a customization block and RSpec tracks their inclusion locations so it can include them in the output for failed shared examples) but I don't think any of those features apply to what rspec-rails does. In addition, shared example groups are slower than modules -- after all, shared example groups that are included via metadata get compiled into a module so that they build off of config.include.

[pirj]

providing better isolated testing and cherry-picking of rspec-rails

I'm out of context here. @JonRowe can you please provide some background on what issues did we have with using AS::Concern in our codebase? I struggle to understand how getting it out of our codebase would help users of RSpec Rails to get rid of this dependency?

[JonRowe]

@pirj It was more about the fact we don't use it in a way that provides us any benefit over standard Ruby inclusion, or our own mechanisms for sharing example code (Although I agree with Myron here that standard Ruby modules would be the way to go), so removing it would make the code a little bit more stand alone from ActiveSupport for little downside to us. As such people who wanted to use rspec-rails in less "full fat rails" ways would be slightly better off, but as you can see from the lack of activity, no-one ever felt very strongly about it.