chore(sdk): partition KFP SDK source code into runtime and non-runtime code

[connor-mccarthy]

Description of your changes:
The first step toward extracting a KFP runtime package with minimal-dependencies.

This PR isolates runtime-required code to the dsl/ subpackage. The dsl/ subpackage has no dependencies on other subpackages of the KFP SDK.

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title
  convention used in this repository.

[google-oss-prow]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[connor-mccarthy]

/test all

[google-oss-prow]

@connor-mccarthy: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipelines-sdk-execution-tests	2846ff4	link	false	/test kubeflow-pipelines-sdk-execution-tests

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[connor-mccarthy]

/assign @chensun

[chensun]

/lgtm
/approve

Thanks!

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun

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