build: generate index.json

build: generate index.json

The index.json file lies next to Packages index files and contains a
json dict with the package architecture and a dict of package names and
versions.

This can be used for downstream project to know what packages in which
versions are available.

Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit 218ce40cd738f3373438aab82467807a8707fb9c)