clonebundles: move a bundle of clone bundle related code to a new module In the process on general clone bundle automatically, we need to make some function available more widely. This is a good opportunity to extract a significant amount of code from `mercurial.exchange` into a new `mercurial.bundlecaches`. This make `mercurial.exchange` move under the 3K line range (hooray…). The module is called `bundlecaches` because I expect it to be eventually useful for more than just clone bundle (like pull bunbles). Differential Revision: https://phab.mercurial-scm.org/D9208

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