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/Cargo.lock

@@ -0,0 +1,29 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "alloc"
+version = "0.0.0"
+dependencies = [
+ "compiler_builtins",
+ "core",
+]
+
+[[package]]
+name = "compiler_builtins"
+version = "0.1.36"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7cd0782e0a7da7598164153173e5a5d4d9b1da094473c98dce0ff91406112369"
+dependencies = [
+ "rustc-std-workspace-core",
+]
+
+[[package]]
+name = "core"
+version = "0.0.0"
+
+[[package]]
+name = "rustc-std-workspace-core"
+version = "1.99.0"
+dependencies = [
+ "core",
+]