package: rework contents of package index.json

package: rework contents of package index.json

Rework the generation of the index.json version of the package
indexes to match the original intent (i.e., for use by the ASU
server and other downstream projects).  The current file contains
package names that have ABI versioning, making them unusable by ASU,
so we now remove the ABI suffixes.

Also adds a 'version' field to the json, so downstream utilities
can detect the new semantics of the package name fields.

Links: https://github.com/openwrt/openwrt/commit/218ce40cd738f3373438aab82467807a8707fb9c
Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19051
Signed-off-by: Robert Marko <robimarko@gmail.com>