Previously, during the build, the full package_index.json was downloaded
and distributed with the Arduino IDE.
This lead to a situation where it was difficult to test new AVR cores
before publishing them to the public package_index.json.
Now the bundled AVR core is specificed in the file:
`hardware/package_index_bundled.json`
this index is loaded from the IDE at startup and the package_index.json
is overlayed on it.
This should also solve part of #5143 (Repeatable builds and snapshots of
package/library indexes)
