repo: make index and index-meta dicts immutable.
repo: make index and index-meta dicts immutable.

For clients that want to modify its content they will need to copy a mutable
dictionary instead. This will avoid unwanted modifications.

datenwolf/xbps,stpx/xbps,datenwolf/xbps,datenwolf/xbps,stpx/xbps,ebfe/xbps,ebfe/xbps,stpx/xbps,ebfe/xbps