Add the pkgutil style init file to datadog_checks_dev for support on IDEs language server

[AAraKKe]

What does this PR do?

This PR bring sback the pkgutil style __init__.py file to datadog_checks_dev that was removed previously in #19826.

Motivation

Language servers like Pylance (used in VS Code) are currently unable to resolve imports from the datadog_checks.dev package. This leads to incorrect import errors within the IDE, which breaks features like autocompletion and "Go to Definition" for test fixtures. This change aims to fix this issue to improve the developer experience.

This problem occurs because of an inconsistency in how the datadog_checks namespace package is defined:

• The datadog-checks-base package uses a legacy pkgutil-style namespace (which requires an __init__.py file).
• The datadog-checks-dev package uses a modern implicit namespace (which omits the __init__.py file).

Static analysis tools see the pkgutil-style __init__.py from the base package and correctly assume the entire datadog_checks namespace must follow that pattern. As a result, they do not discover the dev package, which uses the conflicting implicit method.

At runtime, this works because the code in the pkgutil __init__.py file dynamically extends the package path to include the dev directory. However, this runtime behavior cannot be inferred by static analysis, leading to the errors in the IDE.

From the Native Namespace docs:

It is extremely important that every distribution that uses the namespace package omits the init.py or uses a pkgutil-style init.py. If any distribution does not, it will cause the namespace logic to fail and the other sub-packages will not be importable.

EDIT: After talking with @ofek and @iliakur we decided to go ahead and get rid of the need of using the old pkgutil by removing it from datadog-checks-base. We have been documenting the proper imports for years already and decided it was time to get rid of it.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.92%. Comparing base (382ead7) to head (5246020).
Report is 2 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
ambari	85.75% <ø> (ø)
azure_iot_edge	82.08% <ø> (ø)
btrfs	83.33% <ø> (ø)
cassandra	?
cisco_aci	89.43% <ø> (ø)
cloud_foundry_api	96.11% <ø> (ø)
confluent_platform	?
dcgm	93.54% <ø> (ø)
dns_check	93.84% <ø> (ø)
eks_fargate	94.05% <ø> (ø)
hive	?
hivemq	?
hudi	?
ignite	?
infiniband	92.34% <ø> (ø)
jboss_wildfly	?
kafka	?
kube_controller_manager	97.88% <ø> (ø)
kube_dns	95.94% <ø> (ø)
kubelet	91.09% <ø> (ø)
kubernetes_state	89.49% <ø> (ø)
litellm	95.65% <ø> (ø)
mac_audit_logs	80.79% <ø> (ø)
network	80.86% <ø> (-13.04%)	⬇️
nvidia_nim	93.10% <ø> (ø)
presto	?
riakcs	88.82% <ø> (ø)
slurm	90.86% <ø> (ø)
solr	?
tcp_check	90.72% <ø> (-1.33%)	⬇️
tibco_ems	91.98% <ø> (ø)
tomcat	?
traffic_server	96.13% <ø> (ø)
weblogic	?

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[AAraKKe]

Many of the checks are breaking because they are using relative imports. I know we were working on removing them in #20467 after having move ddev lint and formatter to ruff (in #20451).

I will mark this PR draft until that is finished or it will take a long time to actually get everything refactored.