Fixes #170650

This PR fixes workflows that relied on putting child routes in files different than parent files, which resulted in the (private) mixin being generated in a different file than the route itself.

To avoid releasing a new major version in such a short period, this PR makes the mixin public only if needed. Admittedly makes the behavior somewhat unexpected for the user. @chunhtai @hannah-hyj I will leave that decision for you, whether that's ok or I should make it always public and release a major version instead.

Since this change just fixes a use-case that already didn't compile in 3.x.x, this PR doesn't bump the major version.

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