Move annotations, like @Since / @DeveloperApi, into spark-tags

https://issues.apache.org/jira/browse/SPARK-14613 reminds us that annotations like @Since are only accessible from modules that depend on spark-core right now. Being merely annotations, things like this and @AlphaComponent, etc. can be in a module like spark-tags (renamed from spark-test-tags). This component already has annotations for tests.

I don't propose to move SQL-specific annotations out though, as they're not that general.

I believe we should get this change in for 2.0 since it's a module name and structure change.