Allow `uv` crate to be used as a library (#4642)
This is pulled out of #4632 — a user noted that it would be useful to
use the `uv` crate from Rust. This makes it way easier to invoke `uv`
from Rust with arbitrary arguments as well as use various functionality
in the `uv` crate.
Note this is no longer needed for #4632 and is not particularly urgent.
