Make migrate script (more) idempotent

[strarsis]

Summary

Using the migrate-from-wpackagist.sh script multiple times results in the wp-packages repository being added multiple times to repository field in the composer.json.

composer.json after two invocations of migrate-wpackagist.sh:

{
  "repositories": [
    [...]
    {
      "name": "wp-packages",
      "type": "composer",
      "url": "https://repo.wp-packages.org"
    },
    {
      "name": "wp-packages",
      "type": "composer",
      "url": "https://repo.wp-packages.org"
    }
  ],

Additional context

No response

[retlehs]

Would be a good improvement 👍 I'm curious why you'd run it multiple times?