The `cask` attribute doesn't make as much sense on Linux but can be
ignored there. The advantage of this change is that (like #4086) it
allows figuring out the relevant cask for a formulae requirement on a
Linux machine.
This enhances the Java requirement to support prompting the user
to install the correct legacy Java version via Cask for formulae
that don't yet work with the latest version of Java. Previously,
even if the formula had a strict requirement that a specific,
older version of Java be used, the messaging would tell the user to
`brew cask install java` (i.e. to install the latest version of Java),
which wouldn't actually satisfy the requirement.
Split the core requirement class into generic, Linux-specific,
and macOS-specific parts.
Additionally, the Linux version is now able to detect Java versions
(the previous Linuxbrew implementation was only able to detect
if Java was present at all.)
