Clean up graph solver CLI as externally usable command

[markan]

The graph CLI work is a little rough as it was intended as a tool to test and experiment with the code. However there's interest in the tool as an actual thing, so we're going to take some time to clean it up a little bit and merge.
This includes

1. tests for all of the user accessible commands
   2)better error handling and output (Result, not panic)
   3)some improvements to the input and output to the build order command, to

• filtering to narrow the rebuild to a subset (to speed test cycles)
• make it easy to specify the changed set
• tweak the output to be suitable for the intended user (json, parseable text, etc)

Aha! Link: https://chef.aha.io/features/APPDL-120

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