Update "Developing packages" page to use Swift Package Manager

### Page URL

https://docs.flutter.dev/packages-and-plugins/developing-packages

### Page source

_No response_

### Describe the problem

Our plugin docs should use Swift Package Manager now.

### Expected fix

_No response_

### Additional context

_No response_

### I would like to fix this problem.

- [x] I will try and fix this problem on docs.flutter.dev.