refactor: separate dsc into a library and a binary crate

[EricCrosson]

The motivation for this change is so that I can import the engine of docker-source-checksum as a function in some other binary, without introducing changes to current docker-source-checksum users.

[EricCrosson]

Marking as "ready for review" because from what I can tell, this is working as expected! But it may be in what you consider to be a rough state, happy to iterate on this as feedback is presented

[dpc]

@EricCrosson All good, but I think underscores vs hypens are super confusing. Would putting the binary and library in the same crate (like e.g. here: https://github.com/crev-dev/cargo-crev/blob/e9b35a1d24a7b877bf20d19632e8ef9f04b210ac/cargo-crev/Cargo.toml#L17) work for you?

[EricCrosson]

I think underscores vs hypens are super confusing

Agreed! There may still be a bit of that, but I followed the example you linked and the diff is much cleaner. Thanks for the pointer

[ocornoc]

I have some criticisms of this code (was requested privately by @EricCrosson).

[ericcrosson-bitgo]

Thanks for the input @ocornoc. I'd like to keep this PR as close to a lift-and-shift as possible, but I'm happy to circle back with these suggestions

[dpc]

@EricCrosson Proposed changes looked good so I added them. Please ping me if you're OK with my landing as is.

[EricCrosson]

@dpc Looks good to me! Thank you