Merge cargo-rm into cargo

A scratchpad like #568

Ordered list:
- [x] Update CLI flags to match the merged `cargo-add`
- [x] Add `--target` support (#711)
- [x] Dry-run support
- [x] First letter of warnings/errors should be lower case
- [x] Change `CRATE` in help to `DEP` to clarify that we want the dependency name and not the crate name (in case of a rename)
- [x] Create `merge-rm` branch
- [ ] Refactor to match cargo's directory structure
- [ ] Port tests to snapbox (like #659)
- [ ] Port tests to cargo-test-support
- [ ] Refactor out of `cargo`s `ops/cargo_add/manifest.rs` into a place for reuse
- Possibilities: `ops/cargo_edit` or `edit` and make it public
- [ ] Port to cargo's internals
- [ ] Merge the command (like in rust-lang/cargo#10472)
- [ ] Update documentation (like in rust-lang/cargo#10578)
- [ ] Update completions (like in rust-lang/cargo#10577)