[Merge] ~ubuntu-core-dev/ubuntu-seeds/+git/platform:dd-curtin-requirements into ~ubuntu-core-dev/ubuntu-seeds/+git/platform:cosmic

[Steve Langasek]

Review: Needs Fixing

'Needs fixing' to add grub-pc to the list, per Ryan.
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https://code.launchpad.net/~ubuntu-core-dev/ubuntu-seeds/+git/platform/+merge/356999
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