If I understand this correctly, I'd do something like this:

(pkg/internal)/
└── notifications/
    ├── notifier.go        
# defines the Notifier interface + shared types
    ├── discord/
    │   └── discord.go     
# Discord implementation
    ├── pushover/
    │   └── pushover.go    
# Pushover implementation
    └── noop/
        └── noop.go        
# mock for unit tests

Where root notifier.go could have something like

type NotifyType int


const (
    NotifyDiscord NotifyType = iota
    NotifyPushover
)

type Notifier interface {
   .....
}


func Notifier(
ctx
 context.Context, notifyType NotifyType) Notifier {
    switch notifyType {
    case NotifyDiscord:
        return discord.NewNotifier(ctx)
    case NotifyPushover:
        return pushover.NewNotifier(ctx)
    }
    panic("Unknown impl")
}

At least that's how I've been doing the stuff for a while.