Fix lib list --all not returning library includes in json output

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
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• What kind of change does this PR introduce?

Fixes a regression when calling lib list.

• What is the current behavior?

lib list --all --fqbn arduino:avr:uno --format json doesn't return the provides_includes property for the libraries installed together with the specified core, arduino:avr in this case.

If the includes property found in the library.properties file is empty we would return nothing.

• What is the new behavior?

lib list --all --fqbn arduino:avr:uno --format json now correctly return provides_includes property for all libraries.

Now if the includes property in the library.properties file is empty we walk the library's folders to gather the headers file names.

• Does this PR introduce a breaking change, and is titled accordingly?

Nope.

• Other information:

None.

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