Fix cask migration warnings.

[reitermarkus]

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Readd a separate loader for the default tap to avoid showing a warning when a cask was migrated to it when trying to load a cask using the short token.

Fixes #16653.

[apainintheneck]

LGTM. Nice idea to add tests for to the formula loader too.

[MikeMcQuaid]

Thanks for quick fix @reitermarkus!