Feature request: find orphaned dependencies

[icanhazstring]

Discussed in #75

^{Originally posted by BackEndTea February 11, 2020}

Is your feature request related to a problem? Please describe

As described here: https://twitter.com/icanhazstring/status/1227190874855165953

I require package A. A has a dependency on package B.
Later i remove package A, but B remains in my composer.lock. This can be due to manually editing the composer.lock, merge conflicts, or other reasons.

B is now an orphaned dependency, and using composer why for it even says its not installed.

Describe the solution you'd like

I would like composer unused to find these dependencies, maybe with another command.

Describe alternatives you've considered

I wrote my own 'quick and dirty' implementation right here, which make a graph of the dependencies, and does a (rather unoptimized) DFS to check what dependencies aren't part of the main graph.

Teachability, Documentation, Adoption, Migration Strategy

This should probably be a new command.