[SPARK-13548][BUILD] Move tags and unsafe modules into common

[rxin]

What changes were proposed in this pull request?

This patch moves tags and unsafe modules into common directory to remove 2 top level non-user-facing directories.

How was this patch tested?

Jenkins should suffice.

[rxin]

cc @srowen are there other things we can easily do?

[srowen]

I like this -- I assume common is for modules that don't even depend on Spark Core and could even be separated from Spark. That seems true of both of these, so, OK.

The next change I'd suggest is putting extras/* into external, and likewise for both docker* modules.

The only weird thing there is that java8-tests isn't exactly "external". I suppose it goes away as soon as Spark requires Java 8, so if it's odd, at least it won't last too long.

I'd also like to unify the various "config / build / binary" dirs, but that's going to be a little more disruptive or need more discussion about the theory.

[SparkQA]

Test build #52174 has finished for PR 11426 at commit 0c967cc.

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

[rxin]

"I suppose it goes away as soon as Spark requires Java 8, so if it's odd, at least it won't last too long."

That's probably at least one year away...!

[SparkQA]

Test build #2596 has finished for PR 11426 at commit 0c967cc.

• This patch passes all tests.
• This patch does not merge cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #52248 has finished for PR 11426 at commit f953d03.

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

[SparkQA]

Test build #2597 has finished for PR 11426 at commit 0c967cc.

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

[SparkQA]

Test build #2598 has finished for PR 11426 at commit f953d03.

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

[rxin]

Alright merging this in master.