When determining macOS requirements for a cask, we may need to
reference requirements from related on_system blocks (e.g.,
`on_monterey :or_older`, `on_ventura`, `on_sonoma :or_newer`) when
`depends_on macos` isn't adequate.
Sometimes casks specify different `depends_on macos` values in macOS
on_system blocks but that value is only set when the cask is loaded
in an environment that satisfies the on_system block's requirements.
There are other casks that contain macOS on_system blocks and use
`depends_on macos` outside of the on_system blocks but it may only
use the macOS version of the lowest on_system block (e.g.,
`>= :monterey`), which isn't sufficient if the cask's values vary
based on macOS version.
To be able to simulate macOS versions that meet the requirements of
all the on_system blocks in a cask, we need to collect the macOS
requirements in a way that doesn't require OS simulation. This is also
something that's easy to do in on_system methods, so this adds a
`macos_requirements` array to `UsesOnSystem`, containing
`MacOSRequirement` objects created from the cask's macOS on_system
block conditions.
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