refactor: Made command functions to access PackageManager unavailable from public API

[cmaglie]

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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?

This refactoring aims to make the following functions of the commands package private:

commands.GetPackageManagerExplorer(...)
commands.GetPackageManager(...)
commands.GetLibrariesManager(...)

This change should clarify that the arduino package must not be used from the cli package (to avoid bypassing the gRPC API).
It also highlights where incorrect access to the arduino package is made, because those places are now reported as compile-time errors:

package github.com/arduino/arduino-cli
	imports github.com/arduino/arduino-cli/internal/cli
	imports github.com/arduino/arduino-cli/internal/cli/board
	imports github.com/arduino/arduino-cli/internal/cli/arguments
	internal/cli/arguments/completion.go:23:2: use of internal package github.com/arduino/arduino-cli/commands/internal/instances not allowed
package github.com/arduino/arduino-cli
	imports github.com/arduino/arduino-cli/internal/cli
	imports github.com/arduino/arduino-cli/internal/cli/board
	imports github.com/arduino/arduino-cli/internal/cli/arguments
	internal/cli/arguments/port.go:24:2: use of internal package github.com/arduino/arduino-cli/commands/internal/instances not allowed

The first one tries to list the available upload protocols. The second one performs a board watch to get available ports.

Both have been updated to use only the public gRPC API.

What is the current behavior?

No changes.

What is the new behavior?

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

No

Other information

Once, completed this PR should fix #1948

[codecov]

Codecov Report

Attention: 45 lines in your changes are missing coverage. Please review.

Comparison is base (1e51cdc) 62.82% compared to head (6c6db14) 62.77%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2335      +/-   ##
==========================================
- Coverage   62.82%   62.77%   -0.06%     
==========================================
  Files         204      205       +1     
  Lines       19311    19283      -28     
==========================================
- Hits        12132    12104      -28     
  Misses       6119     6119              
  Partials     1060     1060              

Flag	Coverage Δ
unit	62.77% <68.30%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
commands/board/list.go	58.70% <100.00%> (ø)
commands/board/listall.go	91.13% <100.00%> (ø)
commands/compile/compile.go	66.78% <100.00%> (ø)
commands/core/download.go	57.14% <100.00%> (ø)
commands/core/install.go	71.42% <100.00%> (ø)
commands/core/list.go	86.95% <100.00%> (ø)
commands/core/search.go	84.21% <100.00%> (ø)
commands/core/uninstall.go	50.00% <100.00%> (ø)
commands/core/upgrade.go	80.64% <100.00%> (ø)
commands/debug/debug_info.go	66.86% <100.00%> (ø)
... and 19 more

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[umbynos]