Reorganize Spark version specific code structure

[jerrychenhf]

To act as the template using profile based compile time source code organization, we need a more standard way to organize the version specific source code. Here is the proposal:

project-root
--src
----main -> common code entry
----test -> common test code entry
--spark-3.0.1 -> version specific entry
----main -> version specific code entry
----test -> version specific test code entry
--spark-3.1.1
----main
----test
...

And we will use build-helper-maven-plugin to add additional version specific source code to build.
Additionally, as part of this refactor, change maven-scala-plugin (old) to scala-maven-plugin (new)