Use absolute path in external config glob when looking for external configuration files in Elastic Agent

[kvch]

What does this PR do?

In #30087 we introduced loading inputs configuration from external folders in Agent. This PR changes the path to the external configuration files from relative to absolute.

Why is it important?

Using absolute path makes the solution bit more stable. The feature works even without this change unless the user change path.config or the folder. But having this in covers all "cases".

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.

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label. Could you fix it @kvch? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Reason: null

• Start Time: 2022-02-02T08:37:35.369+0000

• Duration: 114 min 29 sec

• Commit: 0e73e21

Test stats 🧪

Test	Results
Failed	0
Passed	7248
Skipped	16
Total	7264

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