subcommands: fetch_repo and fetch_srclibs

These are the last two subcommands needed for the complete build cycle on both production and verificaiton. @uniqx implemented them a while ago, they have been running for months, and recently the last known bugs were fixed. This merge request is @uniqx's code, renamed to follow the subcommand naming scheme and reformatted to fit the new subcommand coding standards (which still need to be documented).