rustbuild: Implement `make dist`

This commit implements the `make dist` command in the new rustbuild build
system, porting over `dist.mk` and `prepare.mk` into Rust. There's a huge amount
of complexity between those two files, not all of which is likely justified, so
the Rust implementation is *much* smaller.

Currently the implementation still shells out to rust-installer as well as some
python scripts, but ideally we'd rewrite it all in the future to not shell out
and be in Rust proper.