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.)
This will allow us to have global setup and teardown for tests.
For example, we can automatically clear caches after each test, to avoid
annoying intermittent failures like #1879 and #1886.
