index: add json index writing

[xnox]

YOLO - no tests neither before nor after

Doing update on a single package, of an apk repository, seems to generate json which is much larger, so the json seems very stale.

$ git diff --stat
 APKINDEX.json | 179154 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------------------------
 1 file changed, 114348 insertions(+), 64806 deletions(-)

Once this change lands, we can update melange index command in pipelines to generate new.json in addition to new.tar.gz and then copy that to the buckets.

Melange Pull Request Template

Functional Changes

• This change can build all of Wolfi without errors (describe results in notes)

Notes:

SCA Changes

• Examining several representative APKs show no regression / the desired effect (details in notes)

Notes:

Linter

• The new check is clean across Wolfi
• The new check is opt-in or a warning

Notes: