fix [std_instead_of_core]: don't assume alloc/core are in the extern prelude

[bushrat011899]

Objective

• Fixes std-instead-of-core: core not in scope #15836
• Fixes std-instead-of-alloc: there is no alloc::collections::VecDeque #16695

Solution

The current version of this lint doesn't consider the extern prelude before emitting a suggestion, which can lead to fix failures. For example:

#![warn(clippy::std_instead_of_alloc)]

// The should not be a suggestion, since `alloc` is not in scope.
use std::vec::Vec;
//~^ std_instead_of_alloc

This currently emits a failing fix, since it suggests replacing std::vec::Vec with alloc::vec::Vec, but alloc is not available. This PR addresses this by demoting suggestions to helps if alloc/core aren't obviously available. "Obvious" means:

• core is available if no_core and no_implicit_prelude aren't used or extern crate core; is present at the crate root (and thus added to the extern prelude)
• alloc is available if extern crate alloc; is present at the crate root (and thus added to the extern prelude)

Renaming crates, or non-root usage of extern crate is not considered for brevity. Since the lint is always emitted, the worst case scenario is a suggestion isn't automatically applicable.

---

Notes

• No AI tooling of any kind was used during the creation of this PR.
• Based on fix [std_instead_of_core]: false positive for multi-imports #17252 (first commit). Please only review the second commit in this PR until fix [std_instead_of_core]: false positive for multi-imports #17252 is merged.

---

Please write a short comment explaining your change (or "none" for internal only changes)

changelog: fix certain false positive suggestions for [std_instead_of_core] when alloc or core aren't in the extern prelude.

[rustbot]

r? @Jarcho

rustbot has assigned @Jarcho.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: 8 candidates
• 8 candidates expanded to 8 candidates
• Random selection from Jarcho, dswij, llogiq, samueltardieu

[bushrat011899]

@rustbot blocked

#17252 should be merged first.

[github-actions]

Lintcheck changes for e34e2dc

Lint	Added	Removed	Changed
clippy::std_instead_of_alloc	3	1	462
clippy::std_instead_of_core	9	4	4

This comment will be updated if you push new changes

[bushrat011899]

The large quantity of changes in std_instead_of_alloc is expected, as alloc is usually only explicitly included when a crate is already aiming for no_std compatibility. So there are likely many instances of the suggestion being made, when it should only be a help level emission.