This commit stabilizes the `std::sync::atomics` module, renaming it to
`std::sync::atomic` to match library precedent elsewhere, and tightening
up behavior around incorrect memory ordering annotations.

The vast majority of the module is now `stable`. However, the
`AtomicOption` type has been deprecated, since it is essentially unused
and is not truly a primitive atomic type. It will eventually be replaced
by a higher-level abstraction like MVars.

Due to deprecations, this is a:

[breaking-change]

These methods were intended to be stable as of rust-lang#16258 but the tags have since
been lost in various refactorings. This commit re-adds the `#[stable]`
attributes to each of these functions.

…, r=huonw

 These methods were intended to be stable as of rust-lang#16258 but the tags have since
been lost in various refactorings. This commit re-adds the `#[stable]`
attributes to each of these functions.

These methods were intended to be stable as of rust-lang#16258 but the tags have since
been lost in various refactorings. This commit re-adds the `#[stable]`
attributes to each of these functions.

internal: Rewrite `ImportMap::search_dependencies`

Second attempt, this time with a lot of perf improvements

Fixes rust-lang/rust-analyzer#16080
Fixes rust-lang/rust-analyzer#16074
Fixes rust-lang/rust-analyzer#15845

The main bottleneck for flyimport completions are now import path calculation and completion item rendering.