Fix core search command not searching through package terms

[silvanocerza]

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

Fixes core search command.

• What is the current behavior?

core search command doesn't search in a Platform's Package name, maintaner or website fields.
arduino-cli core search retro --additional-urls https://www.retrokits.com/rk002/arduino/package_retrokits_index.json doesn't find any core.

arduino-cli core search rk --additional-urls https://www.retrokits.com/rk002/arduino/package_retrokits_index.json finds the expected Retrokit core.

• What is the new behavior?

core search command now searches in a Platform's Package name, maintaner or website fields.
Now both arduino-cli core search retro --additional-urls https://www.retrokits.com/rk002/arduino/package_retrokits_index.json and arduino-cli core search rk --additional-urls https://www.retrokits.com/rk002/arduino/package_retrokits_index.json find the expected Retrokit core.

• Does this PR introduce a breaking change?

No.

• Other information:

None.

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