`Missing reference` for unexported Base symbols

[simonmandlik]

If Base defines a symbol, but doesn't export it, importing it with using is a valid command. But Language Server yields a warning

Here, AbstractVecOrTuple and CodeUnits are from Base, but are not exported.

LS produces a false positive warning. If importing unexported stuff from modules is a bad practice, shouldn't the Warning say something different?

Maybe related to https://github.com/julia-vscode/LanguageServer.jl/issues/1204 and https://github.com/julia-vscode/LanguageServer.jl/issues/1197