all-java: add provides for jre, jre-headless and jdk

This MR adds provides to all java packages, so that depending packages do not require specific java version. This way you can use (e.g.) maven with java 11 only and you won't need java 8 to be installed (but unused).

This is only the first part, adding provides for:

• java-jre-headless: A headless version of java
• java-jre: A java version with GUI support
• java-jdk: A java version including the compiler (development version)

The next MR will update the applications which require java (e.g. maven and ant) to use these provides-abstractions. A quick grep over the aports yielded the following packages:

community/apache-ant
community/chromium
community/cloudi
community/commons-daemon
community/elasticsearch
community/erlang
community/gdal
community/geany
community/gradle
community/icedtea-web
community/java-jansi-native
community/java-jffi
community/java-jna
community/java-lz4
community/java-postgresql-jdbc
community/java-sigar
community/java-snappy
community/jenkins
community/jetty-runner
community/jruby
community/kodi
community/libkml
community/libreoffice
community/maven
community/py3-yuicompressor
community/R
community/ruby-concurrent-ruby
community/sigar
community/tomcat-native
non-free/unifi
non-free/yed
testing/artifactory-oss
testing/async-profiler
testing/bazel2
testing/bazel3
testing/belle-sip
testing/clojure
testing/closure-compiler
testing/eclipse-ecj
testing/grpc-java
testing/java-asmtools
testing/java-jtharness
testing/java-jtreg
testing/leiningen
testing/liquibase
testing/logstash
testing/ma1sd
testing/multimc
testing/octave
testing/openfire
testing/sbt
testing/spark
testing/tomcat9

This MR will also close #5760 (closed)