[v2] tools: fix fixmodules script

[rarguelloF]

What does this PR do?

The tools/fixmodules script is used to fix the modules in the repository recursively, adding replace directives for imports of other modules in the same repository + using the same go version as the root module.

It had a bug where it was injecting some unnecessary replace since to the logic of inspecting go source files and checking import statement was broken.

Motivation

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.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: rarguelloF/fix-fixmodules-script
Commit report: 8299646
Test service: dd-trace-go

✅ 0 Failed, 4084 Passed, 64 Skipped, 2m 44.36s Total Time