fix: workaround for otel agent dependency causes orchestrion failure

[eliottness]

What does this PR do?

Moved the blank import of go.opentelemetry.io/collector/pdata/pprofile which was introduced to avoid go mod tidy from failing to resolve a coherent dependency closure into a new file guarded by a build tag so that it is not present in builds under normal circumstances.

This removes a dependency cycle caused by the pprofile package having a transitive dependency on net/http, while the
gopkg.in/DataDog/dd-trace-go.v1/internal package is imported some code that is injected by orchestrion into net/http itself (resulting in a cycle, breaking builds).

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: eliottness/fix-the-fix-of-otel
Commit report: 7d002ee
Test service: dd-trace-go

✅ 0 Failed, 5121 Passed, 70 Skipped, 2m 53.05s Total Time