<!--
Thanks for sending a pull request!

Here are some tips for you:
  1. If this is your first time, please read our contributor guidelines: https://kyuubi.readthedocs.io/en/latest/community/contributions.html
  2. If the PR is related to an issue in https://github.com/NetEase/kyuubi/issues, add '[KYUUBI #XXXX]' in your PR title, e.g., '[KYUUBI #XXXX] Your PR title ...'.
  3. If the PR is unfinished, add '[WIP]' in your PR title, e.g., '[WIP][KYUUBI #XXXX] Your PR title ...'.
-->

### _Why are the changes needed?_
<!--
Please clarify why the changes are needed. For instance,
  1. If you add a feature, you can talk about the use case of it.
  2. If you fix a bug, you can clarify why it is a bug.
-->
Make Kyuubi jars clear.

### _How was this patch tested?_
Manual test.  After this PR, the packaged dir looks like:
![image](https://user-images.githubusercontent.com/12025282/118102872-e3895400-b40b-11eb-994c-26d28b45e9a8.png)

Closes #634 from ulysses-you/extension-dir.

Closes #634

2c5009a [ulysses-you] comment
35063ea [ulysses-you] Merge branch 'master' of https://github.com/NetEase/kyuubi

 into extension-dir
f2b4fff [ulysses-you] init
Authored-by: ulysses-you <ulyssesyou18@gmail.com>
Signed-off-by: Kent Yao <yao@apache.org>