Refactor searchplugins.json

Main level keys:
* locales
* images
* metadata

For each *locale* inside *locales*, the structure is the same as
before: product -> channel.

For each channel 2 keys:
* searchplugins
* p12n

All searchplugins are stored inside *searchplugins*, not at the same
level of p12n as it used to be.