Consider breaking some of the utils out into separate modules

[wraithan]

Currently building a subcommand, would be nice to be able to link in small libraries for things that subcommand authors will want to mirror from cargo's core commands. In this case I needed to link cargo in for cargo::util::important_paths::find_project_manifest to make sure my behavior matches what cargo does, even if cargo changes the behavior over time. Which adds a ton of time to my cold builds because cargo is large and has lots of deps.

I imagine there are likely other utilities that will make more sense as separate modules too, which is why I made the issue a bit more generic.

[wraithan]

And https://travis-ci.org/wraithan/cargo-bump/builds/99808557 is an example of why. Needed a similar function for finding something else so I can drop my cargo dep, but it is a shame I can't just use the implementation from cargo without extra considerations.