Fix empty name on manually installed platforms without platform.txt

[silvanocerza]

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• 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)

• What kind of change does this PR introduce?

Bug fix.

• What is the current behavior?

If a platform installed manually in the Sketchbook hardware folder doesn't have a platform.txt file its name will be empty.

$ arduino-cli core list                                                                                                                                                                                     
ID                            Installed Latest Name                                                                                                                                                 
arduino-beta-development:samd                                                                                                                                                                       
arduino:avr                   1.8.3     1.8.3  Arduino AVR Boards

The above arduino-beta-development:samd platform is installed in ~/Arduino/hardware/arduino-beta-development/samd.

• What is the new behavior?

If a platform installed manually in the Sketchbook hardware folder doesn't have a platform.txt file its name will be composed by the packager name and the architecture separated by an hyphen (-).

$ arduino-cli core list
ID                            Installed Latest Name                         
arduino:avr                   1.8.3     1.8.3  Arduino AVR Boards           
arduino-beta-development:samd                  arduino-beta-development-samd

The above arduino-beta-development:samd platform is installed in ~/Arduino/hardware/arduino-beta-development/samd, so arduino-beta-development is assumed to be the packager and samd the architecture.

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

Nope.

• Other information:

This is the same behaviour of the Java IDE and affects the gRPC interface so it will be fixed in the IDE 2 too.

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[ubidefeo]

works as expected