Clean old dependencies from system

[krisgesling]

When a Skill is installed it can define system packages, Python packages and other Mycroft Skills as dependencies. These are installed automatically but not removed if the Skill is uninstalled at a later date.

We cannot just remove any dependencies on uninstall as these may be required by other Skills or services.

One suggestion from MycroftAI/skill-date-time#90 is to perform some type of system cleaning at regular intervals such as major releases. This may be possible on a Mycroft only system, but may be more challenging on a shared system like a desktop install unless its a Snap container or similar. Even if we only cleaned Python packages from the Mycroft venv it would be an improvement on the current system.

[forslund]

For a desktop install there are two ways to go. virtual-environment like in mycroft-desktop-deb or the skills are installed using the system package manager like the implementation @PureTryOut has made.

In the first case the major version cleanup will work. In the second the system package manger will manage the dependencies so it's a non-issue.

[PureTryOut]

Indeed, when managed with a system package manager this is a non-issue. I hope that use-case will remain supported and some automatic cleanup by msm can be disabled in some way.

[forslund]

Thinking around it. msm would only do cleanup when doing an upgrade or a remove and not automatically. The mycroft package for the Mark-X's or the desktop-deb would handle any scheduled cleanup so it shouldn't affect the installs handled by distro package managers. (you should be safe)

A possibility is to have a msm clean command. Parse the skills requirements and add the ones by core then remove any package that isn't on that list or required by a package on that list. A bit of tree-building but it should be doable.