Mirror repository

Like the current dist-dump [1] it would be nice to clone whole repositories, so that one can act completely indepedent from any external service even when installing dependencies as "source".
- Reduce latency and traffic
- Independency
- As a sideeffect reading the `composer.json`s and dumping the dist-files should be much faster when done from the local clone :smile:
- Includes: Later updates will be much faster, because only a `git fetch --all`, or `svn update` (or similar) is required

[1] http://getcomposer.org/doc/articles/handling-private-packages-with-satis.md#downloads

Start by reading the current dist-dump flow and the Satis documentation linked in the issue, then trace how composer.json files and dist files are read. Define how whole-repository cloning and later git fetch or svn update operations should integrate with source dependency installation; done means repositories can be used independently of external services and updates avoid full reprocessing.

Written by the indexing model from the issue text.