computeoutgoing: move the function from 'changegroup' to 'exchange' Now that all users are in exchange, we can safely move the code in the 'exchange' module. This function is really about processing the argument of a 'getbundle' call, so it even makes senses to do so.

getchangegroup: take an 'outgoing' object as argument (API) There is various version of this function that differ mostly by the way they define the bundled set. The flexibility is now available in the outgoing object itself so we move the complexity into the caller themself. This will allow use to remove a good share of the similar function to obtains a changegroup in the 'changegroup.py' module. An important side effect is that we stop calling 'computeoutgoing' in 'getchangegroup'. This is fine as code that needs such argument processing is actually going through the 'exchange' module which already all this function itself.