New Gradel structure - reuse tar file

[markflyhigh]

This experimental is based on a assumption that py2 and py3 sdist works the same and the tar file can be used inter-changeable.

• only build sdist once (so put task in sdks/python/build.gradle). This task should execute first before parallel execution of many test tasks. The generated tar file can be used in all downstream tasks (e.g. tox tests and IT tests).
• move copy sources out of sdist and only tox tasks depend on it
• re-organized and refactor tasks, params in BeamModulePlugin to make code cleaner.

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Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---			---	---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[markflyhigh]

test test