Add ExternCrateDecl to HIR
Adding these doesn't really require much design effort as they represent a single import, unlike use trees which are one item that represent 0 or more imports.
We only resolve to this definition when actually resolving on the name or alias of an `extern crate name as alias` item, not usages yet as that requires far more changes that won't lead anywhere without giving it more thought. Nevertheless the changes slightly improve IDE things, an example being hover on the decl showing the merged doc comments for example.
cc https://github.com/rust-lang/rust-analyzer/issues/14079
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