exchange: move bundle specification parsing from cmdutil Clone bundles require a well-defined string to specify the type of bundle that is listed so clients can filter compatible file types. The `hg bundle` command and cmdutil.parsebundletype() already establish the beginnings of a bundle specification format. As part of formalizing this format specification so it can be used by clone bundles, we move the specification parsing bits verbatim to exchange.py, which is a more suitable place than cmdutil.py. A subsequent patch will refactor this code to make it more appropriate as a general API.