Get slave-side pottery code onto the build-farm slave

The slave-side pottery code must be installed on the slave systems.

The easiest way to get this done is to move it into the launchpad-buildd package, which is built from lib/canonical/buildd. That will ensure that it is installed automatically on any build-farm slave.

Some manual testing will be needed to make sure the paths are right; we can't automate realistic integration-testing between the server and build-farm slaves. To get a realistic slave environment, build the .deb in lib/canonical/buildd and install it. This does mess with your system a bit (creating a new user etc.), so that's one reason for installing it in a chroot. Another reason is to avoid hiding dependencies that might not be satisfied on the real slaves.