> All "compilers" have multiple steps usually invoking other programs at some point: parsing, actual code transformation, linking, etc..., some also find packages, some rely on an external tool or long command line invocations.

There is no technical reason the compiler can do the job of build system, but they are typically separated because of separation of concerns. Rustc needs not be tightly coupled with Cargo---it just has to understand enough of package concepts (`--extern`) for the actual compilation, so they can be independently managed and evolve. Odin's would be the polar oppsite, while Zig's approach is somewhere in the middle (compiler-as-a-library).