Auto merge of #37732 - jseyfried:use_extern_macros, r=nrc

Support `use`ing externally defined macros behind `#![feature(use_extern_macros)]`

With `#![feature(use_extern_macros)]`,
 - A name collision between macros from different upstream crates is much less of an issue since we can `use` the macros in different submodules or rename with `as`.
 - We can reexport macros with `pub use`, so `#![feature(macro_reexport)]` is no longer needed.
 - These reexports are allowed in any module, so crates can expose a macro-modular interface.

If a macro invocation can resolve to both a `use` import and a `macro_rules!` or `#[macro_use]`, it is an ambiguity error.

r? @nrc

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