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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)

• What kind of change does this PR introduce?

update

• What is the current behavior?

When users run a command requiring package_index.json to be already there (for example core install arduino:[email protected]) the CLI was giving this error: Error installing: There were errors loading platform indexes
#792

• What is the new behavior?

Now it should download automatically the package_index.json when it's not already in place and it is required by the command the user is running.

• Does this PR introduce a breaking change?

no, the user can still use core update-index

• Other information:

See how to contribute