Formulae, casks, and resources have a `#livecheckable?` method that
indicates whether they contain a `livecheck` block. This is intended
to be read as "has a livecheckable?", not "is livecheckable?" (as
livecheck can find versions for some packages/resources without a
`livecheck` block). Unfortunately, correct understanding of this
method's behavior [outside of documentation] relies on historical
knowledge that few people possess, so this is often confusing to
anyone who hasn't been working on livecheck since 2020.
In the olden days, a "livecheckable" was a Ruby file containing a
`livecheck` block (originally a hash) with a filename that
corresponded to a related formula. The `livecheck` blocks in
livecheckable files were integrated into their respective formulae in
August 2020, so [first-party] livecheckables ceased to exist at that
time. From that point forward, we simply referred to these as
`livecheck` blocks.
With that in mind, this clarifies the situation by replacing
"livecheckable" language. This includes renaming `#livecheckable?` to
`#livecheck_defined?`, replacing usage of "livecheckable" as a noun
with "`livecheck` block", replacing "livecheckable" as a boolean with
"livecheck_defined", and replacing incorrect usage of "livecheckable"
as an adjective with "checkable".
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