[FLINK-8264] [core] Add 'scala.' to the 'parent-first' classloading patterns

[StephanEwen]

What is the purpose of the change

Adding scala. to the "parent-first-patterns" makes sure that Scala classes are not duplicated through "child-first" classloading when users accidentally package the Scala Library into the application jar.

Since Scala classes traverse the boundary between core and user space, they should never be duplicated.

Brief change log

• Adds scala. to the default value of classloader.parent-first-patterns.

Verifying this change

This change can be verified as follows:

• Create a very simple quickstart Scala project using a Scala lambda for a filter function (_ => true).
• Package it such that the Scala library is in the user code jar
• Without the fix, you get a weird class cast exception during deserialization, with this fix, everything is fine.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[StephanEwen]

@aljoscha I think you may have an opinion on this one

[aljoscha]

I think this looks good!