### What changes were proposed in this pull request?
Move some classes extending Scala collections into parallel source trees, to support 2.13; other minor collection-related modifications.
Modify some classes extending Scala collections to work with 2.13 as well as 2.12. In many cases, this means introducing parallel source trees, as the type hierarchy changed in ways that one class can't support both.
### Why are the changes needed?
To support building for Scala 2.13 in the future.
### Does this PR introduce any user-facing change?
There should be no behavior change.
### How was this patch tested?
Existing tests. Note that the 2.13 changes are not tested by the PR builder, of course. They compile in 2.13 but can't even be tested locally. Later, once the project can be compiled for 2.13, thus tested, it's possible the 2.13 implementations will need updates.
Closes#26728 from srowen/SPARK-30012.
Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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