This change moves `java_library` targets from the top-level BUILD file
into `//java/{core,lite,util}` and declares `alias` targets to point to
their new locations (hence, this is not a breaking change).

This will allow users that don't use Java to stop depending on
`@rules_java` (e.g. as requested in
bazelbuild/rules_scala#989 (comment)).

Note that there is no intention to deprecate + remove the top-level
targets in the foreseeable future.