Fix core search not showing board name for 3rd party cores

[masci]

Fixes #506
Note: not an issue if additional URL is listed in the configuration file.

core search with no arguments was wrongly reporting additional cores that, when installed, were picked up by the search algorithm but due to the missing ref to the package index had partial metadata, resulting in #506

Unit and integration tests were added to prevent regressions.

[spoenemann]

Following the instructions provided in #506 I get this final output:

ID                   Version   Name                                             
Arrow:samd           2.1.0     Arrow Boards                                     
Intel:arc32          2.0.4     Intel Curie Boards                               
Intel:i586           1.6.7+1.0 Intel i586 Boards                                
Intel:i686           1.6.7+1.0 Intel i686 Boards                                
Microsoft:win10      1.1.2     Windows 10 Iot Core                              
arduino:avr          1.8.2     Arduino AVR Boards                               
arduino:mbed         1.1.3     Arduino nRF528x Boards (Mbed OS)                 
arduino:megaavr      1.8.5     Arduino megaAVR Boards                           
arduino:nrf52        1.0.2     Arduino nRF52 Boards                             
arduino:sam          1.6.12    Arduino SAM Boards (32-bits ARM Cortex-M3)       
arduino:samd         1.8.4     Arduino SAMD Boards (32-bits ARM Cortex-M0+)     
arduino:samd_beta    1.6.25    Arduino SAMD Beta Boards (32-bits ARM Cortex-M0+)
atmel-avr-xminis:avr 0.6.0     Atmel AVR Xplained-minis                         
emoro:avr            3.2.2     EMORO 2560                                       
industruino:samd     1.0.1     Industruino SAMD Boards (32-bits ARM Cortex-M0+) 
littleBits:avr       1.0.0     littleBits Arduino AVR Modules

The arbotix:avr core is now missing from the list, and the Boards Manager of the IDE doesn't show it either. Is this intended?

[masci]

@spoenemann yes it is. Even if the core is installed, the command requires the additional url pointing to the additional package index file to show additional cores.
You can pass the additional url to the CLI via config settings, like this (not tested):

_, err := client.SetValue(context.Background(),
		&settings.Value{
			Key:      "board_manager",
			JsonData: `{"additional_urls": ["https:/.example.com"]}`,
		})

[rsora]

LGTM

[spoenemann]

When is 0.8.0 going to land?