rename-dependency doesn't perform hyphen/underscore translation

When renaming a dependency in Cargo.toml (rust-lang/cargo#5422),

This works: `not_a = { path = "../a", package = "a" }`
This doesn't: `not-a = { path = "../a", package = "a" }`; attempting to import the crate results in `can't find crate for not_a`

It's never necessary to use hyphens in a rename, but supporting it would be less surprising.