[DO-NOT-MERGE] Decouple PySpark core API to pyspark.core package

[HyukjinKwon]

What changes were proposed in this pull request?

TBD

Why are the changes needed?

TBD

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing test cases

Was this patch authored or co-authored using generative AI tooling?

No.