GH-612: core search can exact-match Platform#ID too.

[kittaakos]

When matching against a Platform#ID, the match must be a
case-insensitive exact match.

Closes #612.

Signed-off-by: Akos Kitta kittaakos@typefox.io

akos.kitta@Akoss-MacBook-Pro arduino-cli % ./arduino-cli core search                                               
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         1.1.4     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         1.8.5     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                   

akos.kitta@Akoss-MacBook-Pro arduino-cli % ./arduino-cli core search "Arduino SAMD Boards (32-bits ARM Cortex-M0+)"
ID           Version Name                                        
arduino:samd 1.8.5   Arduino SAMD Boards (32-bits ARM Cortex-M0+)

akos.kitta@Akoss-MacBook-Pro arduino-cli % ./arduino-cli core search "arduino:samd"                                
ID           Version Name                                        
arduino:samd 1.8.5   Arduino SAMD Boards (32-bits ARM Cortex-M0+)

akos.kitta@Akoss-MacBook-Pro arduino-cli % ./arduino-cli core search "arduino:sa"                                  
No platforms matching your search.
akos.kitta@Akoss-MacBook-Pro arduino-cli %