Ddtrace now requires patch.py rather than __init__.py files.

[purple4reina]

What does this PR do?

Instead of keeping __init__.py files, keep patch.py files when building for layer releases.

Motivation

This change in dd-trace-py https://github.com/DataDog/dd-trace-py/pull/12153/files#diff-6a70144455f9fea41df44b3e578cfaa81ae9c5297e41cce9ef02a5b719013817R265 now means ddtrace looks for patch.py files. This change is backported to both the v2 and v3 release lines.

Testing Guidelines

The integration tests should cover this.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[joeyzhao2018]

This fix LGTM.
However... i wonder how it was working before?