Remove more Requirement logic to enable future removal of default
formula logic. Also, output deprecations, convert symbol requirement
usage to deps and simplify the compatibility code for the direct
Requirement usage.
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.)
