Refactor sql package structure

[typhoonzero]

c.f. #1553 (comment)

Currently, sql package contains almost all core code for parsing, generating python code and executing. We need to put those features in a separated package structure for better code understanding:

1. parser package is already moved under pkg folder
2. feature_derivation package is already moved under pkg folder
3. tools like pipe, verifier are already moved under pkg folder
4. TODO: move ir to pkg folder
5. TODO: move testdata to pkg folder

We currently have two job execution mode: workflow mode and run in local mode.

• workflow mode, we generate a Couler python program, then execute the python program to generate a YAML file, then call functions under pkg/argo to submit it and monitor the job's status. Each step in the workflow is a repl command.
• repl mode, we generate some actual Python training/predicting code and execute the python code to get the result.

We may need to use a command step instead of repl to be more meaningful. For that w'll have

1. cmd/step calls pkg/step to run a step or, in the future generate a step Python code
2. pkg/step contains:
   1. run current step and get output (will only be used by repl if we generate python code for each step but not using the command step to run a single SQL statement)
   2. pkg/step/codegen generate step python code
3. pkg/workflow contains:
   1. submit workflow and monitor the status
   2. pkg/workflow/codegen generate Couler/Fluid python code
   3. pkg/workflow/argo submit, get status, get logs for argo
   4. pkg/workflow/tekton submit, get status, get logs for tekton

[wangkuiyi]

Related #1434

[typhoonzero]

Related #1583

[Yancey0623]

It seems that we also need a pkg/submitter package:

`-pkg/submiter
    |-python.go    # cmd: python xxx.py
    |-pai.go       # cmd: pai -Djobname=sqlflow_job ...
    |-alisa.go     # goalisa: alisa.createTask('pai -Djobname=sqlflow_job ...')
    |- TODO: alps/elasticdl ....

[typhoonzero]

@Yancey1989 I recommend put all current "submitter" to pkg/step and rename the interface to Executor, so we can call the executors like step.Executor.ExecuteTrain(...) etc. It's more meaningful.

[typhoonzero]

Related: #1434