[breaking] refactor: made some golang modules private (part 3)

[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?

In preparation for the release of 1.0.0, we will make most of the root packages internal and expose only the packages commands and rpc (that are 1:1 correlated with the gRPC API).

• Made client_example example internal inside rpc package
• Mode docsgen internal
• Made i18n library internal (this one may be a candidate to became a standalone library)

What is the current behavior?

What is the new behavior?

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

Yes

Other information

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b9476de) 67.62% compared to head (a3bef94) 67.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2467      +/-   ##
==========================================
- Coverage   67.62%   67.61%   -0.02%     
==========================================
  Files         204      207       +3     
  Lines       20562    20710     +148     
==========================================
+ Hits        13905    14003      +98     
- Misses       5519     5563      +44     
- Partials     1138     1144       +6     

Flag	Coverage Δ
unit	67.61% <ø> (-0.02%)	⬇️

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[cmaglie]

The failing markdown link-check is a false negative.
The missing link https://github.com/arduino/arduino-cli/blob/master/rpc/internal/client_example will become valid after merging the PR.