Make loads from @rules_rust//rust:defs.bzl come out on one line

[dtolnay]

At first I was confused why some of the generated files were coming out with loads formatted like this:

rules_rust/crate_universe/3rdparty/crates/BUILD.anyhow-1.0.68.bazel

Lines 9 to 13 in 4ef3d4a

	load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
	load(
	"@rules_rust//rust:defs.bzl",
	"rust_library",
	)

Surely this should be as follows? The wrapped line is even shorter than the unwrapped line.

load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
load("@rules_rust//rust:defs.bzl", "rust_library")

It turned out to be because crate_universe unconditionally generates all the loads that might be needed:

load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
load(
    "@rules_rust//rust:defs.bzl",
    "rust_library",
    "rust_binary",
    "rust_proc_macro",
)

and then Buildifier deletes any that are unused, but without rewrapping any lines:

load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
load(
    "@rules_rust//rust:defs.bzl",
    "rust_library",
)

This PR tweaks crate_universe to generate only loads that are used, so that we retain control over how they are wrapped. After this PR, single loads get put on one line while multiple loads get put on multiple lines.

rules_rust/crate_universe/3rdparty/crates/BUILD.anyhow-1.0.68.bazel

Lines 9 to 10 in e987f20

	load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
	load("@rules_rust//rust:defs.bzl", "rust_library")

rules_rust/bindgen/3rdparty/crates/BUILD.bindgen-0.60.1.bazel

Lines 9 to 14 in e987f20

	load("@rules_rust//cargo:defs.bzl", "cargo_build_script")
	load(
	"@rules_rust//rust:defs.bzl",
	"rust_binary",
	"rust_library",
	)