Improved package index merging logic

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
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• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

In some sporadic cases, there may exist some stale package_*_index.json files with an invalid or empty entry in the tools section. In particular, due to the way the package indexes are loaded, the tool resource metadata may be overwritten or deleted by an invalid index. This change should improve the resilience of the Arduino CLI in most of these cases.

What is the current behavior?

What is the new behavior?

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

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.66%. Comparing base (863c1ec) to head (d350fdc).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2713      +/-   ##
==========================================
- Coverage   67.68%   67.66%   -0.02%     
==========================================
  Files         234      234              
  Lines       22226    22226              
==========================================
- Hits        15043    15040       -3     
- Misses       6001     6003       +2     
- Partials     1182     1183       +1     

Flag	Coverage Δ
unit	67.66% <100.00%> (-0.02%)	⬇️

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