add external macro checks to iter_without_into_iter and into_iter_without_iter

[y21]

Fixes #12037

I think it's useful to still lint on local macros, since the user should still be able to add another impl with the IntoIterator or iter method. I think it's also fairly common to write a macro for generating many impls (e.g. for many similar types), so it'd be nice if we can continue linting in those cases.
For that reason I went with in_external_macro.

I also added a test for #[allow]ing the lint while I was at it.

changelog: [iter_without_into_iter]: don't lint if the iter method is defined in an external macro
changelog: [into_iter_without_iter]: don't lint if the IntoIterator impl is defined in an external macro

[rustbot]

r? @Jarcho

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[Jarcho]

Thank you. @bors r+

[bors]

📌 Commit 0848e12 has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0848e12 with merge c992247...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing c992247 to master...