This patch fixes a compilation / build break in Spark's `java8-tests` and refactors their POM to simplify the build. See individual commit messages for more details.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #12073 from JoshRosen/fix-java8-tests.

## What changes were proposed in this pull request?

Move `docker` dirs out of top level into `external/`; move `extras/*` into `external/`

## How was this patch tested?

This is tested with Jenkins tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #11523 from srowen/SPARK-13595.