Remove hardcoded builtin-tools and move them to package_index.json

[ubidefeo]

At the moment arduino-cli installs default tools at first launch

• https://github.com/arduino/arduino-cli/blob/master/commands/bundled_tools_ctags.go
• https://github.com/arduino/arduino-cli/blob/master/commands/bundled_tools_mdns_discovery.go
• https://github.com/arduino/arduino-cli/blob/master/commands/bundled_tools_serial_discovery.go

These should be converted to standalone tools and added to the main package_index.json to be downloaded at need

[silvanocerza]

PR that updates package_index.json created: https://github.com/bcmi-labs/package_index/pull/7

[silvanocerza]

Draft PR that removes hardcoded builtin package from Arduino CLI created: #1537
Will be ready for review after bcmi-labs/package_index#7 is merged and a new package_index.json is released.