Downgrade all external packages to the official ones

Solution 1:

You could use apt-forktracer

To install it you must first enable the Universe Repository and then install it either via the software center or with the command sudo apt-get install apt-forktracer

Here's an excerpt from the man page:

apt-forktracer analyzes each installed package separately, reporting on the standard output these packages which are in a "non-standard" state. What "non-standard" means depends on the mode of program operation:

default (non-verbose) mode
    this state means packages in an incorrect state (e.g. no candidate version) or packages whose candidate version is different than the newest available official version.

verbose mode
    this state also includes packages whose installed version is different from the candidate version 

And here's an example of the default output:

libavformat57 (7:3.1~~git20160413.62652~ubuntu14.04.1) [LP-PPA-motumedia-ffmpeg-daily: 7:3.1~~git20160413.62652~ubuntu14.04.1]
libavresample3 (7:3.1~~git20160413.62652~ubuntu14.04.1) [LP-PPA-motumedia-ffmpeg-daily: 7:3.1~~git20160413.62652~ubuntu14.04.1]
libavfilter6 (7:3.1~~git20160413.62652~ubuntu14.04.1) [LP-PPA-motumedia-ffmpeg-daily: 7:3.1~~git20160413.62652~ubuntu14.04.1]

For more information see man apt-forktracer

Now that we know the package names in question we can remove (or purge them)

Other methods as @muru notes in his comment are to:

A) use synaptic to locate the obsolete packages.

You can then select those you wish to remove or completely remove as shown below:

Or

B) use aptitude

Upon launching aptitude Select Actions -> Become root and after entering your password Select "Obsolete and Locally Created Packages" as shown below:

Navigate to the package you want to remove as shown below:

Press the - key to mark the package for removal. When you've completed matking the packages you wish to remove press g to remove them Or is you've made a mistake in marking, Ctrlu to Undo yiour changes for the session.

Sources:

https://manpages.debian.org/testing/apt-forktracer/apt-forktracer.8.en.html

http://aptitude.alioth.debian.org/doc/en/ch02s01s02.html