bottle: check actual bottle block contents when --keep-old

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This PR restores some of the brew bottle --merge --write --keep-old logic that was removed in #10102

Instead of checking for default spec values, we can check which methods were being called in the old bottle block

Supersedes #10135

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Review period will end on 2021-01-01 at 17:58:13 UTC.

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Review period ended.

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Merging now so @iMichka can continue work on the new cellar syntax—I can open a follow-up PR if any changes are requested after merge