Some bugfix in profile parsing / Better support for platforms having tools dependencies on other platforms.

[cmaglie]

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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)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

1. Fix a profile sketch.yml rendering bug.
2. Fix a bug that prevents extracting all the needed information in installed.json.

What is the current behavior?

1. It will print an empty libraries: YAML object when the profile has no libraries. This is invalid YAML.
2. If the installed platform depends on a tool from another platform, the tool metadata is not extracted in the installed.json file. This would eventually make the platform unusable if the original index is removed.

What is the new behavior?

1. The libraries: field is not printed if there are no libraries in the profile.
2. The installed.json now has all the necessary metadata.

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

No

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 95.06173% with 4 lines in your changes missing coverage. Please review.

Project coverage is 67.89%. Comparing base (4891ded) to head (5e15ea3).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/arduino/cores/packageindex/index.go	94.66%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2942      +/-   ##
==========================================
+ Coverage   67.82%   67.89%   +0.07%     
==========================================
  Files         238      238              
  Lines       22460    22489      +29     
==========================================
+ Hits        15234    15270      +36     
+ Misses       6023     6019       -4     
+ Partials     1203     1200       -3     

Flag	Coverage Δ
unit	67.89% <95.06%> (+0.07%)	⬆️

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