What does this PR do?

This PR decouples dependency version (what is called in the packaging code BEAT_VERSION) from SNAPSHOT and VERSION_QUALIFIER specified for elastic-agent package version.

What works:

• building an alpha1 version of 9.0.0 starting from SNAPSHOT manifest

  MANIFEST_URL="https://snapshots.elastic.co/9.0.0-7ab33a91/manifest-9.0.0-SNAPSHOT.json" AGENT_DROP_PATH=build/elastic-agent-drop AGENT_PACKAGE_VERSION="9.0.0-alpha1" BEAT_VERSION="9.0.0-SNAPSHOT" VERSION_QUALIFIER="alpha1" PLATFORMS="linux/amd64" mage -v clean downloadManifest package
  

• building an alpha1 version of 9.0.0 starting from 8.17.1 staging manifest

  MANIFEST_URL="https://staging.elastic.co/8.17.1-f2b43bf6/manifest-8.17.1.json" AGENT_DROP_PATH=build/elastic-agent-drop AGENT_PACKAGE_VERSION="9.0.0-alpha1" BEAT_VERSION="8.17.1" VERSION_QUALIFIER="alpha1"  mage -v clean downloadManifest package
  

What doesn't work:

• packaging a dev elastic agent using external SNAPSHOT dependencies

  EXTERNAL=true PLATFORMS="linux/amd64" mage -v clean  package
  

  The calculated name of artifacts contains -SNAPSHOT-SNAPSHOT and needs to be fixed

Why is it important?

Checklist

• I have read and understood the pull request guidelines of this project.
• 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/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...