packetbeat: reduce levels of indirection, simplify and document

[efd6]

What does this PR do?

This clarifies the intentions of the code by impoving source locality, removing unneeded indirection and adding documentation.

Why is it important?

This will help in maintenance and when adding new features.

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. Not user-facing

Author's Checklist

• [ ]

How to test this PR locally

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @efd6? 🙏.
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]

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• Start Time: 2022-08-23T04:46:00.786+0000

• Duration: 45 min 33 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1673
Skipped	19
Total	1692

💚 Flaky test report

Tests succeeded.

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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[andrewkroh]

++ godocs ✍️

[andrewkroh]

Maybe this file should be decoders.go now?

[efd6]

Will do.