Add support in apt & synaptic to install "custom" packages via source

Users that install their own versions of a software package currently have no facility in apt to inform apt that they've installed their own version (usually via source). This has given rise to a practice of the need being met by installing the apt version of the package, deleting the files installed and locking the version of the installed package... Then installing their own version in to /opt or where ever they want.

The solution could/should involve support for specifying details in apt/synaptic for installing one's own version from source. The CheckInstall utility could be applied to make a meta-package entry in Synaptic/apt.

This blueprint was inspired by: https://answers.launchpad.net/ubuntu/+question/109693

For more on CheckInstall: https://help.ubuntu.com/community/CheckInstall

This blueprint is immediately derived from here: https://blueprints.launchpad.net/ubuntu/+spec/custom-src-install-support
Though I couldn't migrate that Blueprint to this project, so here it is.