fix: scan libraries on install

[Bikappa]

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What kind of change does this PR introduce?

Rescan libraries before installing new ones. Upon each installation request the libraries folders
are rescanned.

What is the current behavior?

The installed libraries are scanned only at initialization time. If other are added
while the program runs, especially in deamon mode, these are not known to the running instance.
This causes the execution to fail improperly if different installation attempts have an
overlap/conflict in dependencies.

What is the new behavior?

The installed libraries are reloaded before every installation request, so the logic has the updated
status.

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

Other information

Fixes #1802

[codecov]

Codecov Report

Base: 36.53% // Head: 36.56% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (6e5a811) compared to base (271d241).
Patch coverage: 83.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2037      +/-   ##
==========================================
+ Coverage   36.53%   36.56%   +0.02%     
==========================================
  Files         229      229              
  Lines       19379    19385       +6     
==========================================
+ Hits         7080     7088       +8     
+ Misses      11463    11462       -1     
+ Partials      836      835       -1     

Flag	Coverage Δ
unit	36.56% <83.33%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
arduino/libraries/librariesmanager/install.go	16.58% <0.00%> (-0.09%)	⬇️
...ino/libraries/librariesmanager/librariesmanager.go	53.27% <100.00%> (+1.99%)	⬆️
arduino/cores/packagemanager/package_manager.go	66.58% <0.00%> (+0.75%)	⬆️

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[umbynos]

code LGTM 🏁