main: unify output for resolve/db commands, make it parseable

[proger]

I also squeezed a tiny cleanup in form of a resolve_and_print function.

I'm not sure if the tool output is supposed to be stable, but this PR makes resolve and db look awk'able (and print the installer by default to save a roundtrip).

[proger]

not sure if both KeyError and ResolutionError handlers are necessary

[wjwwood]

Sorry, I know this issue is over a year old.

I don't think we can do this. I do know of at least a few places the output of db is being used. Perhaps we could introduce an optional --output-version= option. Where by default it would be something like plain_text-v1. And this could be plain_text-v2 or something like that. We could also have something like json-vX in the future which dumped it as some kind of JSON structure. I'm drawing inspiration from brew info --json=... on this idea.

Unfortunately, as-is I don't feel comfortable merging this as it departs from the existing out and rosdep is not tied to ROS versions so it would affect all versions, past and present.

[wjwwood]

Needs to preserve old output, either as the default or somehow able to opt-into the old behavior.