[packages] Add mockito support to `update-dependency`

[stuartmorgan-g]

Now that we have the update-dependency repo tooling command, we need to add support to it to regenerate mockito files, like the support we recently added for regenerating files when updating pigeon. Since we're now pinning mockito, there are more situations where we need to mass-update it, and it's a very manual process (see flutter/packages#3544 for example).

[github-actions]

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