[SPARK-28560][SQL][followup] code cleanup for local shuffle reader

[cloud-fan]

What changes were proposed in this pull request?

A followup of #25295

This PR proposes a few code cleanups:

1. rename the special getMapSizesByExecutorId to getMapSizesByMapIndex
2. rename the parameter mapId to mapIndex as that's really a mapper index.
3. BlockStoreShuffleReader should take blocksByAddress directly instead of a map id.
4. rename getMapReader to getReaderForOneMapper to be more clearer.

Why are the changes needed?

make code easier to understand

Does this PR introduce any user-facing change?

no

How was this patch tested?

existing tests

[cloud-fan]

@JkSelf @xuanyuanking

[SparkQA]

Test build #112111 has finished for PR 26128 at commit 0c0191e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[JkSelf]

LGTM. Thanks @cloud-fan working on this.

[cloud-fan]

thanks for the review, merging to master!