Deploy dependencies atomically and related cleanup subcommand (#768)

Deploy dependencies atomically and related cleanup subcommand (#768)

* Use a temporary to fetch & deploy releases

This is done to avoid an interrupted download (for example with Ctrl-C) to
mistake our deployer, which considers an existing directory as a successfully
deployed dependency.

* Cmd.Clean: implement --temp option

To remove any dangling temporaries

* Improve clean --cache to avoid useless downloads

* Test for `alr clean --temp`