Trim down snapcraft_legacy and plan migration effort

[mr-cal]

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Request

1. Remove lifecycle code from snapcraft_legacy
2. Remove lifecycle unit tests from tests/legacy
3. For the remaining store commands, determine the tasks needed to migrate them out of snapcraft_legacy

The problem it solves

So that snapcraft_legacy can be fully removed from the codebase.

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/SNAPCRAFT-1278.

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