Switch to a build.rs file for dependencies

[brettcannon]

https://doc.rust-lang.org/cargo/reference/build-scripts.html

Since the build requirements are so simple for the non-Rust stuff, having cargo handle those cases via shell executions should be fine. Only trick will be breaking out the code that generate the man page from https://github.com/brettcannon/python-launcher/blob/main/dodo.py, but that can just become a part of https://github.com/brettcannon/python-launcher/tree/main/release.

Otherwise #128 can take over for the other task runner needs.

[brettcannon]

Because build scripts should only write out to OUT_DIR, that means it wouldn't make sense to keep the generated files in the repository. While that might be good, it may also complicate installation scripts which now have a new set of dependencies to have installed to do a build (e.g. dot and pandoc).