Fixed loading of Library given a non-absolute path

[cmaglie]

Givin a relative path is not ideal, but at least allows the CLI to compile the sketch correctly.

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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?

This PR makes the Library loader a bit more resilient by allowing relative paths as input.
Using relative paths may lead to other problems, but at least this change allows the CLI to not bail out with weird errors.

What is the current behavior?

If a relative path is given (via directories configuration in arduino-cli.yaml) in some cases the path composition during compile fails. See #2180 for an example.

What is the new behavior?

The compilation succeeds.

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

No

Other information

Fix #2180

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: +0.07% 🎉

Comparison is base (246adf9) 62.94% compared to head (5a8d5c4) 63.01%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2265      +/-   ##
==========================================
+ Coverage   62.94%   63.01%   +0.07%     
==========================================
  Files         220      220              
  Lines       19540    19546       +6     
==========================================
+ Hits        12299    12317      +18     
+ Misses       6151     6142       -9     
+ Partials     1090     1087       -3     

Flag	Coverage Δ
unit	63.01% <50.00%> (+0.07%)	⬆️

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

Files Changed	Coverage Δ
arduino/libraries/loader.go	91.13% <50.00%> (-1.63%)	⬇️

... and 2 files with indirect coverage changes

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