scrooge_scala_library: Remove `scala` field on provider and instead return jars in JavaInfo provider

[beala-stripe]

In order to support the IntelliJ Bazel plugin, #562 returns an old style provider from scrooge_scala_library. This is to work around an issue where a JavaInfo provider cannot have multiple output jars, so we return a scala provider instead (see PR #562 for an in depth description of the issue and workaround).

Setting multiple output jars on an JavaInfo will be addressed by this fix, which is expected to arrive in 0.17.0. Once this lands, it will allow us to set multiple output jars by merging multiple JavaInfos and we can then get rid of the scala field and set the output jars on JavaInfo instead.

[johnynek]

relates to #466