Fix validation of dimension fields inside objects

[jsoriano]

What does this PR do?

There is a helper to visit a function with all the fields in a package, this helper was not visiting fields nested inside objects. Modify it to visit also nested fields, and "flatten" their names to give better information in error messages.

Why is it important?

• Fixes validation of dimension fields.
• Error messages on fields validation provide now the full name of the field.
• Reduces the risk of incorrectly using this helper again (as was happening to me in Implement package limits #278).

Checklist

• I have added test packages to test/packages that prove my change is effective.
• I have added an entry in versions/N/changelog.yml.

Related issues

• Discovered in Implement package limits #278.

[elasticmachine]

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• Start Time: 2022-02-17T19:27:42.711+0000

• Duration: 3 min 18 sec

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[mtojek]

LGTM