debug: Enforce programmer selection

[cmaglie]

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What kind of change does this PR introduce?

Enforce programmer selection to debug.

What is the current behavior?

The debug command allows running without specifying the -P flag.

What is the new behavior?

$ arduino-cli debug -b arduino:samd:mkr1000 
Error during Debug: Missing programmer
$ arduino-cli debug -b arduino:samd:mkr1000 --info
Error getting Debug info: Missing programmer
$ arduino-cli debug -b arduino:samd:mkr1000 --info --format json
{
  "error": "Error getting Debug info: Missing programmer"
}
$ 

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 ✅

Comparison is base (ef72bde) 64.48% compared to head (ad261d6) 64.51%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2394      +/-   ##
==========================================
+ Coverage   64.48%   64.51%   +0.03%     
==========================================
  Files         207      207              
  Lines       19592    19594       +2     
==========================================
+ Hits        12633    12641       +8     
+ Misses       5867     5863       -4     
+ Partials     1092     1090       -2     

Flag	Coverage Δ
unit	64.51% <100.00%> (+0.03%)	⬆️

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

Files	Coverage Δ
commands/debug/debug_info.go	69.23% <100.00%> (+2.16%)	⬆️

... and 1 file with indirect coverage changes

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[cmaglie]

@kittaakos I'm cherry-picking this one in the 0.35.x branch