Refactor cargo workspace generation so it can be reused for stdlib builds (#119)

Author: dzbarsky

rs/BUILD.bazel

@@ -37,7 +37,7 @@ bzl_library(
     deps = [
         "//rs/private:annotations",
         "//rs/private:cargo_credentials",
-        "//rs/private:cfg_parser",
+        "//rs/private:cargo_workspace_graph",
         "//rs/private:crate_repository",
         "//rs/private:downloader",
         "//rs/private:git_cargo_workspace_repository",
@@ -46,9 +46,6 @@ bzl_library(
         "//rs/private:registry_config_repository",
         "//rs/private:registry_utils",
         "//rs/private:repository_utils",
-        "//rs/private:resolver",
-        "//rs/private:select_utils",
-        "//rs/private:semver",
         "//rs/private:toml2json",
         "@aspect_tools_telemetry_report//:defs.bzl",
         "@bazel_lib//lib:repo_utils",