Fix modified go.sum issue in beat generators

[andrewkroh]

What does this PR do?

There were two issues:

go mod download was run after the newly generated beat was committed. This modified the go.sum because it
adds hashes for the downloaded dependencies. The caused mage check to fail because the git project has
pending changes.

The dependencies of package_test.go were missing from go.sum. When running mage testPackages it
was failing because github.com/blakesmith/ar and github.com/cavaliercoder/go-rpm were missing.

Why is it important?

CI is failing in the generator tests. This was failing when Go 1.17.1 was updated in #27543 (comment).

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.

How to test this PR locally

PLATFORMS=linux/amd64 make -C generator/_templates/beat test test-package

Issues

• Related Generator tests are failing on master #28361

Logs

 >> Testing package contents
 go: updates to go.mod needed; to update it:
 	go mod tidy
 
 package ran for 4m51.60809048s
 Error: running "go test /var/lib/jenkins/workspace/PR-28353-1-ae052b9e-d3a6-4774-8541-442abc6e8666/pkg/mod/github.com/elastic/beats/[email protected]/dev-tools/packaging/package_test.go -root-owner -files /var/lib/jenkins/workspace/PR-28353-1-ae052b9e-d3a6-4774-8541-442abc6e8666/src/beatpath/testmetricbeat/build/distributions/*" failed with exit code 1

>> Testing package contents
# command-line-arguments
../../../pkg/mod/github.com/elastic/beats/[email protected]/dev-tools/packaging/package_test.go:40:2: missing go.sum entry for module providing package github.com/cavaliercoder/go-rpm; to add:
        go mod download github.com/cavaliercoder/go-rpm
FAIL    command-line-arguments [setup failed]

[elasticmachine]

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

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-10-13T01:18:33.525+0000

• Duration: 29 min 54 sec

• Commit: 311122b

❕ Flaky test report

No test was executed to be analysed.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

[andrewkroh]

/test generator