refactor: drop golang.org/x/tools/go/vcs dependency

[kruskall]

Proposed commit message

golang.org/x/tools/go/vcs is deprecated and was initially developed before module support was added. However we are not using vcs.Cmd at all, we're using one method to retrieve the vcs being used. As a quick fix, we can copy the method since it's short and self-contained and we don't need the whole package.

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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[mergify[bot]]

This pull request does not have a backport label. If this is a bug or security fix, could you label this PR @kruskall? 🙏. For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)