Make the crate callable as a library

[nresare]

This change moves most of the functionality in main.rs to lib.rs and makes main() call it from there. The effect is that the functionality of this crate is available as a library crate as well as built into a binary.

Unfortunately, the github review UI does not make this super easy to review. I found it the easiest to review this by checking out the project twice and manually diff main.rs from origin/main against lib.rs in the head branch. Briefly, the change consists of updating the signature of run() to take args as an argument, and making relevant structs publicly visible.

The reason why I'm looking for this change is that I'm working with some code to build the source files needed to create policy compliant sources files for deb packages for Ubuntu, and with this change I can pull in the vendor-filterer functionality without needing to have separate instructions on how to install the tool.

There are some usability changes that would be useful that I would be interested in contributing as follow-up PRs if you are interested in merging this change, moving the parsing of Cargo.toml config values out of the path of execution when using it as a library comes to mind, but I would not want to make this change bigger than it already is, and I would like to hear some about how you feel about landing this before I go through the work of making those changes

[cgwalters]

Thanks for this PR! Sorry for the delay in review. This makes sense to me overall...I just want to do a bit more sanity checking before merging.