[skip-changelog] Override default pluggable monitor for serial protocol

[MatteoPologruto]

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

Code imperfection fix

What is the current behavior?

Platform authors cannot use a custom monitor tool for the serial protocol.

$ echo "pluggable_monitor.pattern.serial=foobar" >> ~/.arduino15/packages/arduino/hardware/avr/1.8.3/platform.txt
$ arduino-cli monitor --port /dev/ttyACM0 --fqbn arduino:avr:leonardo --protocol serial
Connected to /dev/ttyACM0! Press CTRL-C to exit.
Hello world from the Arduino Leonardo's serial port!
^C

$ echo "pluggable_monitor.pattern.myprotocol=bazqux" >> ~/.arduino15/packages/arduino/hardware/avr/1.8.3/platform.txt
$ arduino-cli monitor --port /dev/ttyACM0 --fqbn arduino:avr:leonardo --protocol myprotocol
Error getting port settings details: Port monitor error: exec: "bazqux": executable file not found in $PATH

The command is also expected to fail when --protocol serial is used.

What is the new behavior?

Platform authors can now use a custom monitor tool for the serial protocol.

$ echo "pluggable_monitor.pattern.serial=foobar" >> ~/.arduino15/packages/arduino/hardware/avr/1.8.3/platform.txt
$ arduino-cli monitor --port /dev/ttyACM0 --fqbn arduino:avr:leonardo --protocol serial
Error getting port settings details: Port monitor error: exec: "foobar": executable file not found in %PATH%

The command now fails as expected.

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

No

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.08 ⚠️

Comparison is base (2c2a5cc) 62.51% compared to head (ac10a73) 62.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2153      +/-   ##
==========================================
- Coverage   62.51%   62.43%   -0.08%     
==========================================
  Files         224      224              
  Lines       19520    19520              
==========================================
- Hits        12203    12188      -15     
- Misses       6222     6233      +11     
- Partials     1095     1099       +4     

Flag	Coverage Δ
unit	62.43% <100.00%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
arduino/cores/packagemanager/loader.go	73.37% <100.00%> (ø)

... and 2 files with indirect coverage changes

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