[WIP] Use some javac classes

[mickaelistria]

This requires to start the VM with ```
--add-opens java.base/java.lang=ALL-UNNAMED
--add-opens jdk.compiler/com.sun.tools.javac.jvm=ALL-UNNAMED --add-opens jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED --add-opens jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED --add-opens jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED

settings.

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[mickaelistria]

Unfortunately, since I am not a committer, my changes to Jenkinsfile are ignored. Can some committer add extra power to this PR? Or maybe re-create this PR on their name? cc @jjohnstn

[mickaelistria]

This is not relevant any more, as other PRs exist to use Javac.