[SPARK-25926][CORE] Move config entries in core module to internal.config.

[ueshin]

What changes were proposed in this pull request?

Currently definitions of config entries in core module are in several files separately. We should move them into internal/config to be easy to manage.

How was this patch tested?

Existing tests.

[ueshin]

cc @jiangxb1987 @cloud-fan @gatorsmile

[SparkQA]

Test build #98388 has finished for PR 22928 at commit 6537102.

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

[SparkQA]

Test build #98389 has finished for PR 22928 at commit e081cb2.

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

[SparkQA]

Test build #98390 has finished for PR 22928 at commit 9d89b0d.

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

[vanzin]

I don't like stashing everything in package.scala; I'm ok-ish with moving them under the internal.config package, but it would be better to keep them in separate source files.

[gatorsmile]

Keeping them in separate source files is also fine to me. I think we should put them in the same package.

[SparkQA]

Test build #98459 has finished for PR 22928 at commit 6144e01.

• This patch fails due to an unknown error code, -9.
• This patch merges cleanly.
• This patch adds no public classes.

[ueshin]

Jenkins, retest this please.

[SparkQA]

Test build #98463 has finished for PR 22928 at commit 6144e01.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[cloud-fan]

LGTM

[cloud-fan]

retest this please

[SparkQA]

Test build #98471 has finished for PR 22928 at commit 6144e01.

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

[dongjoon-hyun]

+1, LGTM.

[cloud-fan]

thanks, merging to master!