Add scp source handlers

[ghost]

Not all source lives in VCS, it is useful to be able to pull the source to be built from an existing repository, e.g. the Debian archive, and rebuild it for a particular architecture or in a particular environment. This could be done with apt-get -d source foo ; dpkg-source -x

It would also be useful to have an scp method which is similar to the current wanna-build in Debian which responds to a source package upload by making the newly uploaded source package available to buildd clients over scp. This allows pybit clients to potentially use the same package uploads before that source becomes available through the main archive, i.e. incoming.debian.org

See next issue for how these can be configured and initialised

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apt handler now implemented.