Zhen Li

- Disallow init accessors in extensions. - Allow init accessor to be `internal`. Resolves #71578

[AST] Fix best import kind when value decl is a typealias of an existential type

6

Resolves #72724 . Fixed behaviors: ```sh /Users/lizhen/Projects/swift-projects/build/Ninja-DebugAssert/swift-macosx-arm64/bin/swift-frontend ./moduleB.swift -I . -L . -l./moduleA -typecheck ./moduleB.swift:3:17: error: type alias 'TypeAlias' (aka 'any Protocol') cannot be imported as 'protocol' 1 │ 2...