[SPARK-5248] [SQL] move sql.types.decimal.Decimal to sql.types.Decimal

[adrian-wang]

@rxin follow up of #3732

[SparkQA]

Test build #25537 has started for PR 4041 at commit 7777a58.

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[SparkQA]

Test build #25537 has finished for PR 4041 at commit 7777a58.

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• This patch adds no public classes.

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Test FAILed.
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[SparkQA]

Test build #25542 has started for PR 4041 at commit aa3d738.

• This patch merges cleanly.

[SparkQA]

Test build #25542 has finished for PR 4041 at commit aa3d738.

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• This patch merges cleanly.
• This patch adds no public classes.

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[adrian-wang]

retest this please.

[SparkQA]

Test build #25547 has started for PR 4041 at commit aa3d738.

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[SparkQA]

Test build #25547 has finished for PR 4041 at commit aa3d738.

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

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[rxin]

import out of order here...

[rxin]

Thanks. Going to merge this.