Don't use RSpec.configure to use the matcher DSL

[alyssais]

rspec-expectations shouldn't have knowledge of how to hook itself into rspec-core. rspec-core already includes the RSpec::Matchers module in example groups, so just including the module there (in addition to extending it) is enough to provide it to rspec-core, even if rspec-core hasn't been loaded yet and is loaded after rspec-expectations.

Fixes rspec/rspec#122.

[myronmarston]

As the failing build indicates, this is not quite sufficient. In rspec-core, we include RSpec::Matchers but we do not extend it (nor should we: that would add a ton of methods to a context where we do not want those methods added!).

I think the only solution here is to update rspec-core to extend RSpec::Matchers::DSL. This can be done in the config.expect_with method, as part of the branch for rspec.

Once that's been done, I think we can remove the extend call being done in rspec-expectations and everything should continue working.

[pirj]

Are you still up to fix this @alyssais ?

[JonRowe]

I'm closing this due to inactivity during the monorepo migration, the issue will be transferred