This adds a `UsesOnSystem` class to `OnSystem`, containing boolean
instance variables to indicate which types of on_system methods are
used in a formula or cask. This is intended as a replacement for
`@on_system_blocks_exist`, which doesn't allow us to determine what
kinds of on_system calls were used. This provides more granularity
but we can still use `@uses_on_system.present?` to determine whether
any on_system calls were used (and this doubles as a `nil` check in
`Formula`, as the `self.class` instance variable has to use a nilable
type).
The `UsesOnSystem` instance variables cover the current
`ARCH_OPTIONS` and `BASE_OS_OPTIONS`. At the moment, we mostly need
to tell whether there are macOS/Linux or Intel/ARM on_system calls,
so I've omitted instance variables for specific macOS version until
we have a need for them.
As a practical example, if you wanted to determine whether a cask
uses Linux on_system calls, you can call
`cask.uses_on_system.linux?`. The `linux` boolean will be `true` if
the cask has an `on_linux` block, an `on_system` block (which requires
Linux), or uses `os linux: ...`. This is something that would be
challenging to determine from outside of `OnSystem` but it's
relatively easy to collect the information in `OnSystem` methods and
make it available like this.
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