fallback to local homebrew formula metadata during migrate

zb migrate currently relies on the live homebrew formula api. that means locally installed formulas that brew still knows about, but the api no longer serves, get skipped.

example: #338, where fasd exists locally and brew info works, but zerobrew can't resolve it from the api.

we should consider a migrate-only fallback to local .brew/*.rb metadata from the installed cellar when the live api misses. this should not block the rest of migration if the local metadata still isn't usable.