x/tools/gopls: gopls restart needed after changing build tags

With gopls on master (9580c84d5753ec8548a100560e8a2d94aeb99972), starting with:

```
-- foo.go --
package main

func main() {}
-- bar.go --
package main

func main() {}
```

1. Confirm you get "duplicate main" diagnostic
2. Add "//go:build ignore" to one of the files
3. Observe you still see the diagnostic until you restart gopls