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.)
It made less sense to call a method `java_version` when it returns
boolean value.
ClosesHomebrew/homebrew#45501.
Signed-off-by: Xu Cheng <xucheng@me.com>
Dependency is another similar, related class and it's super confusing
to have some Requirements that are named *Dependency.
ClosesHomebrew/homebrew#38891.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
