From the packages I've seen, the convention seems to be to have a language-$lang package including the syntax definitions (some also have snippets), and another package for completion, debugging etc.
Atom has recently launched some support for these types of language integration packages, https://ide.atom.io/, building on top of the language server protocol. Packages that support this are then called ide-$lang (Sunsetting Atom - The GitHub Blog).
I'd love to have a proper Atom IDE package for Swift. There are some LSP implementations out there, GitHub - owensd/swift-langsrv: The language service implementation for VS Code. and GitHub - RLovelett/langserver-swift: A Swift implementation of the open Language Server Protocol. (see also VS Code language protocol - #6 by tonyarnold), but I'm not sure how many of the LSP features they support.