Make @opt_out rrule(...) automatically qualify rrule namespace as ChainRulesCore.rrule

[mzgubic]

Closes #545

[codecov-commenter]

Codecov Report

Merging #546 (e31e979) into main (1d076fa) will increase coverage by 0.04%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main     #546      +/-   ##
==========================================
+ Coverage   93.41%   93.45%   +0.04%     
==========================================
  Files          15       15              
  Lines         865      871       +6     
==========================================
+ Hits          808      814       +6     
  Misses         57       57              

Impacted Files	Coverage Δ
src/rule_definition_tools.jl	96.38% <92.85%> (+0.11%)	⬆️
src/projection.jl	97.30% <0.00%> (+0.01%)	⬆️

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[oxinabox]

Make @opt_out not evil

This claim is to strong, there is still all kinds of evil 😂

But more seriously for our own future reference it would be good for this PR to have clearer title and description.

As i understand it the core of the problem was if the name rrule did not refer to ChainRulesCore.rrule but to some other function we would not overload ChainRulesCore.rrule but rather that function from the current module.
So this makes it such that we automatically qualify that name

[oxinabox]

I think

Suggested change

	:(ChainRulesCore.no_rrule)
	:($ChainRulesCore.no_rrule)

might work to actually make this function even if the module name ChainRulesCore isn't in the caller's scope.

If it does then these test could be moved to the isolated macro testing module.
(or a similar setup could be made for this file for testing purposes)
See the setup in
https://github.com/JuliaDiff/ChainRulesCore.jl/blob/main/test/rule_definition_tools.jl#L286

[oxinabox]

I trust you to make appropriate changes then merge when happy