(RE-2157) Add support for arbitrary deb mirrors to packaging 

This PR leverages the work in a recent change to the pbuilderrc to
allow for overriding the secondary mirror to build against. It adds a
new build parameter to Pkg::Config, deb_build_mirrors, which should be
an array of deb sources to inject into the build using the BUILDMIRROR
environment variable. The dist can be templated in using __DIST__.

With tests!
Perhaps a longer message detailing this would be helpful here, e.g. expanding on that the user isn't supposed to replace \__DIST__ with "squeeze" etc.