Use the package_xxx_index.json stored with the boards.txt

[jantje]

Currently Sloeber stores the package_XX_index.json file with the boards.txt but does not use it.
Sloeber searches the json file using the boards manager. If the third party url is removed from the boards manager or the provider of the json file no longer references the installed platform version sketches based on this platform version will no longer compile.
(both have happened already quite some time and each time I am struck with a "what the hell is going on" moment 😒 )

I think it is time to use the stored json file

[jantje]

I don't really use the package_xxx_index.json but by storing the outcome of the parsing in the sloeber.txt file it get close.
Using package_xxx_index.json itself is hard so I'm ok with this way