[breaking] refactor: made some golang modules private

[cmaglie]

Please check if the PR fulfills these requirements

See how to contribute

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• 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).

What is the current behavior?

What is the new behavior?

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

Yes

Other information

[cmaglie]

executils should be promoted to a stand-alone library. I'm converting this PR to a draft until it's done.

$ go mod tidy
go: found github.com/arduino/arduino-cli/executils in github.com/arduino/arduino-cli/executils v0.0.0-00010101000000-000000000000
go: github.com/arduino/arduino-cli/internal/mock_serial_discovery imports
	github.com/arduino/pluggable-discovery-protocol-handler/v2 tested by
	github.com/arduino/pluggable-discovery-protocol-handler/v2.test imports
	github.com/arduino/arduino-cli/executils: module ./internal/executils: reading internal/executils/go.mod: open /home/cmaglie/Workspace/arduino-cli/internal/executils/go.mod: no such file or directory

UPDATE:
executils is now part of go-paths-helper

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (5935e45) 67.55% compared to head (c4256ec) 67.61%.

Files	Patch %	Lines
...builder/internal/diagnostics/compiler_detection.go	0.00%	1 Missing ⚠️
...lder/internal/preprocessor/arduino_preprocessor.go	0.00%	1 Missing ⚠️
commands/debug/debug.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2447      +/-   ##
==========================================
+ Coverage   67.55%   67.61%   +0.06%     
==========================================
  Files         211      204       -7     
  Lines       20712    20566     -146     
==========================================
- Hits        13992    13906      -86     
+ Misses       5581     5521      -60     
  Partials     1139     1139              

Flag	Coverage Δ
unit	67.61% <76.92%> (+0.06%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.