Fix alloc_instead_of_core FP when alloc is an alias

[profetia]

Closes #15579

changelog: alloc_instead_of_core fix FP when alloc is an alias

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[github-actions]

Lintcheck changes for 8f3c6ac

Lint	Added	Removed	Changed
clippy::alloc_instead_of_core	0	1	0
clippy::std_instead_of_core	0	0	2

This comment will be updated if you push new changes

[zihan0822]

I kind of think we should suggest using core::alloc::Layout instead of skipping this lint for alloc::Layout.

[Jarcho]

That's not going to be easily doable. std::alloc is a module actually defined in std which re-exports everything in alloc::alloc. std::fmt, std::collections and others have the same issue.

[Jarcho]

The only thing that needs to be checked is that the first segment resolves to a crate. This should be done before matching on the name since std and alloc both have this issue.

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[profetia]

@Jarcho Updated. Thank you!

[Jarcho]

Thank you.

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