Add runfiles crate

[dzbarsky]

the runfiles crate on crates.io is 3 years old. This PR allows downstream users to depend on runfiles via a git override in their Cargo.toml, so they can build both with Cargo and bazel

[illicitonion]

LGTM! Do you want to wire up CI publishing like we have in

rules_rust/.github/workflows/cargo_bazel_release.yaml

Lines 43 to 47 in 2c3469e

	- name: Publish to crates.io
	run: cargo publish --token ${CRATES_TOKEN}
	working-directory: ./crate_universe
	env:
	CRATES_TOKEN: ${{ secrets.CRATES_TOKEN }}

?