Some formulae/casks contain duplicate checkable URLs or contain
URLs that become duplicates after `#preprocess_url` is used. With
the former, a formula's `stable` and `head` URLs can be the same if
they both use the Git repository. With the latter, a formula's
`homepage` and `stable` URLs are different but `#preprocess_url` can
return the same Git repository URL for each (which can also be
a duplicate of the `head` URL).
The `fabric-completion` formula is a worst case scenario, as it
contains both of these conditions but the repository has no tags.
By default, livecheck would needlessly check the repository three
times (i.e., no versions are found so livecheck tries the next URL,
which happens to be the same URL).
This commit avoids duplicate URLs in `#checkable_urls` and keeps
track of checked URLs in `#latest_version` to avoid a duplicate
caused by `#preprocess_url`. This should effectively resolve the
issue of checking the same URL more than once for a given
formula/cask. Checking the same URL only once across all the
formulae/casks in a given livecheck run will be handled in a later
commit (though I've done most of the groundwork already in previous
PRs).
This aligns `PageMatch` with how cask strategies handle requirements
in their `#find_versions` methods. This moves related logic from
`Livecheck#latest_version` into the `PageMatch` strategy, which feels
a bit more appropriate.
The `ExtractPlist` strategy doesn't return certain values (e.g.,
`:url`), so it's necessary to also ensure these values are present
before printing related debug info. Without these changes, we end up
printing info for a blank value (e.g., "URL (strategy):").
Up to this point, we've had to rely on making `Strategy` constants
private to ensure that the only available constants are strategies.
With the current setup, the existence of a constant that's not a
strategy would break `Strategy#strategies` and
`Livecheck#livecheck_strategy_names`.
Instead, we can achieve the same goal by skipping over constants
that aren't a class. Other than saving us from having to make these
constants private, this is necessary to be able to create a
`Strategy` constant that can be used in all strategies.
