[cleanup] Remove operator reference in /api folder, refactor v2alpha files

[swang392]

What does this PR do?

Moved:

• some consts and a v2alpha1/utils from api/datadoghq/v2alpha1 to a new directory pkg/constants/.
• v2alpha1/test to pkg/testutils
• v2alpha1/datadogagent_default to a new folder, internal/controller/defaults

Motivation

We want to create a new go module for /api, so files that are not specific to the CRDs should be moved out of the /api folder. There are some files in /api that depended on datadog-operator/pkg/defaulting -- this dependency should be removed because a project depending on the /api module would indirectly depend on datadog-operator too.

Additional Notes

Minimum Agent Versions

Are there minimum versions of the Datadog Agent and/or Cluster Agent required?

• Agent: vX.Y.Z
• Cluster Agent: vX.Y.Z

Describe your test plan

Make sure the binary builds and runs as expected. The change is noop.

Checklist

• PR has at least one valid label: bug, enhancement, refactoring, documentation, tooling, and/or dependencies
• PR has a milestone or the qa/skip-qa label

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.84729% with 47 lines in your changes missing coverage. Please review.

Project coverage is 48.94%. Comparing base (efaa4e9) to head (f2d0ac3).

Files with missing lines	Patch %	Lines
pkg/constants/utils.go	65.38%	8 Missing and 1 partial ⚠️
...adogagent/component/clusterchecksrunner/default.go	30.00%	7 Missing ⚠️
internal/controller/testutils/agent.go	0.00%	6 Missing ⚠️
...nal/controller/datadogagent/feature/apm/feature.go	60.00%	3 Missing and 1 partial ⚠️
...controller/datadogagent/component/agent/default.go	80.00%	3 Missing ⚠️
...ller/datadogagent/defaults/datadogagent_default.go	95.08%	3 Missing ⚠️
...nternal/controller/datadogagent/override/global.go	25.00%	3 Missing ⚠️
...oller/datadogagent/component/clusteragent/utils.go	33.33%	2 Missing ⚠️
pkg/testutils/builder.go	50.00%	2 Missing ⚠️
internal/controller/datadogagent/common/utils.go	0.00%	1 Missing ⚠️
... and 7 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1576   +/-   ##
=======================================
  Coverage   48.94%   48.94%           
=======================================
  Files         227      227           
  Lines       20635    20635           
=======================================
  Hits        10100    10100           
  Misses      10010    10010           
  Partials      525      525           

Flag	Coverage Δ
unittests	48.94% <76.84%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ontroller/datadogagent/controller_reconcile_ccr.go	65.34% <100.00%> (ø)
...ontroller/datadogagent/controller_reconcile_dca.go	52.63% <100.00%> (ø)
...controller/datadogagent/controller_reconcile_v2.go	51.89% <100.00%> (ø)
...atadogagent/feature/admissioncontroller/feature.go	66.26% <100.00%> (ø)
...roller/datadogagent/feature/autoscaling/feature.go	72.91% <100.00%> (ø)
...al/controller/datadogagent/feature/cspm/feature.go	75.20% <100.00%> (ø)
...nal/controller/datadogagent/feature/cws/feature.go	79.55% <100.00%> (ø)
...ntroller/datadogagent/feature/dogstatsd/feature.go	68.42% <100.00%> (ø)
...ller/datadogagent/feature/enabledefault/feature.go	36.62% <100.00%> (ø)
...er/datadogagent/feature/eventcollection/feature.go	62.31% <100.00%> (ø)
... and 26 more

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[levan-m]

See this discussion here #1562 (comment).
Given there are api->controller dependencies not considered earlier, instead of eliminating the dependencies we should just move everything not related to CRDs to the main module now - things like env vars, constants, some utilities (secrets.go, condition.go, utils.go, etc).
cc @celenechang

[levan-m]

this is a non-functional change and can be merged without approvals from other teams.