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.py

@@ -0,0 +1,45 @@
+import os
+import toml
+
+rust_src = os.environ['RUSTC_SRC']
+orig_cargo = os.environ['ORIG_CARGO'] if 'ORIG_CARGO' in os.environ else None
+
+base = {
+  'package': {
+    'name': 'alloc',
+    'version': '0.0.0',
+    'authors': ['The Rust Project Developers'],
+    'edition': '2018',
+  },
+  'dependencies': {
+    'compiler_builtins': {
+      'version': '0.1.0',
+      'features': ['rustc-dep-of-std', 'mem'],
+    },
+    'core': {
+      'path': os.path.join(rust_src, 'libcore'),
+    },
+  },
+  'lib': {
+    'name': 'alloc',
+    'path': os.path.join(rust_src, 'liballoc/lib.rs'),
+  },
+  'patch': {
+    'crates-io': {
+      'rustc-std-workspace-core': {
+        'path': os.path.join(rust_src, 'tools/rustc-std-workspace-core'),
+      },
+    },
+  },
+}
+
+if orig_cargo is not None:
+  with open(orig_cargo, 'r') as f:
+    src = toml.loads(f.read())
+    if 'profile' in src:
+      base['profile'] = src['profile']
+
+out = toml.dumps(base)
+
+with open('Cargo.toml', 'x') as f:
+  f.write(out)