Make java library earlier on classpath

[fzakaria]

Description

I have a scala_library with some java files where I would like to override the class for a particular dependency (i.e. something from @maven//).

This is a common idiom for monkey patching some functionality.

In order for this to work, the Java file needs to be first on the classpath. @rules_scala seems to be placing the generated java_library at the end of the CLASSPATH.

This change makes it so that it's first.

[fzakaria]

This patch properly lets me override the Java files of my Maven dependencies from my scala_library

[mbland]

Please add a test for this specific case, and I'd be happy to merge it.

[vinnybod]

I have opened a new PR for this change with a test added https://github.com/bazel-contrib/rules_scala/pull/1797