* replaces openjdk. enables to install multiple versions of openjdk.
* uses tzdata2018g-rearguard, this is what's used upstream.
* adds jtreg and enables some tests.
* split in base, jre and default packages.
base is for the sdk, provides java:environment, exposes JDK8_HOME.
jre is for the runtime, provides java:runtime, exposes JRE8_HOME.
default provides symlinks in /bin and JAVA_HOME.
* switches to main, hotspot and jdk github repos.