build-support/rust: Organize

- `toRustTarget` and friends pulled out from rust tools into rust
   library. Since they don't depend on any packages they can be more
   widely useable.

 - `build-rust-package` gets its own directory

 - `fetch-cargo-tarball` gets its own directory

(cherry picked from commit 18ed048c7b27e288a6c9ba894790a7e67ed5080d)

pkgs/build-support/rust/build-rust-package/sysroot/update-lockfile.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p python3 python3.pkgs.toml cargo
+
+set -e
+
+HERE=$(dirname "${BASH_SOURCE[0]}")
+NIXPKGS_ROOT="$HERE/../../../.."
+
+# https://unix.stackexchange.com/a/84980/390173
+tempdir=$(mktemp -d 2>/dev/null || mktemp -d -t 'update-lockfile')
+
+cd "$tempdir"
+nix-build -E "with import (/. + \"${NIXPKGS_ROOT}\") {}; pkgs.rustPlatform.rustcSrc.override { minimalContent = false; }"
+RUSTC_SRC="$(pwd)/result" python3 "$HERE/cargo.py"
+RUSTC_BOOTSTRAP=1 cargo build || echo "Build failure is expected. All that's needed is the lockfile."
+
+cp Cargo.lock "$HERE"
+
+rm -rf "$tempdir"
+
+