Ast.ScopeDefSourceInside a scope, a definition can refer either to a scope def, or a subscope def
type t = | Var of Scopelang.Ast.ScopeVar.t| SubScopeVar of Scopelang.Ast.SubScopeName.t * Scopelang.Ast.ScopeVar.tIn this case, the Uid.Var.t lives inside the context of the subscope's original declaration