This:

* Moves statements included simply to guard against build errors for
  unused labels to a separate section
* Wraps statements simply included to avoid including unused labels with
  an `if false { ... }` to avoid `go vet` determining that subsequent
  lines won't be hit
* Wraps final rule with a `if true { ... }` to avoid `go vet`
  determining that the `panic` will never be hit if the user includes
  a `return` statement in their final rule

These changes together trick `go vet` into not identifying the
unreachable code though this is purely because it isn't intelligent
enough to notice the static conditionals.

At least in this simple program:

```
package main

import "fmt"

func main() {
  if true {
    fmt.Println("vim-go")
  }
}
```

The Go 1.13.4 compiler elides the conditional.