Cherry-pick #15957 to 7.6: Fix packaging of OSS light modules

[jsoriano]

Cherry-pick of PR #15957 to 7.6 branch. Original message:

What does this PR do?

Move light modules packaging code in Metricbeat to common methods for X-Pack and OSS, so they are properly packaged in both cases.

Add a check to OSS metricbeat packaging test to check that it includes at least one light module.

Why is it important?

Metricbeat 7.6.0 includes OSS light modules, but they are not being packaged.

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

How to test this PR locally

• Check that OSS Metricbeat packages include OSS light modules, but not X-Pack light modules.
• Check that X-Pack Metricbeat packages include OSS and x-pack light modules.

Packages can be built with mage package

Related issues

• Needed to release [Metricbeat] Add Kafka JMX metricsets #14330