Added cargo_manifest_dir submodule to cargo_build_script_runner. (#3161)

This change aims to isolate some of the newer, more complicated parts of
`cargo_build_script_runner`.

Author: UebelAndre

cargo/cargo_build_script_runner/BUILD.bazel

@@ -1,28 +1,30 @@
 load("//rust:defs.bzl", "rust_binary", "rust_library", "rust_test")
 
 rust_library(
-    name = "cargo_build_script_output_parser",
-    srcs = ["lib.rs"],
+    name = "cargo_build_script_runner",
+    srcs = [
+        "cargo_manifest_dir.rs",
+        "lib.rs",
+    ],
     edition = "2018",
 )
 
 rust_test(
     name = "test",
-    crate = ":cargo_build_script_output_parser",
+    crate = ":cargo_build_script_runner",
     edition = "2018",
 )
 
 rust_binary(
-    name = "cargo_build_script_runner",
+    name = "runner",
     srcs = ["bin.rs"],
     edition = "2018",
     visibility = ["//visibility:public"],
-    deps = [":cargo_build_script_output_parser"],
+    deps = [":cargo_build_script_runner"],
 )
 
 rust_test(
     name = "bin_test",
-    crate = ":cargo_build_script_runner",
+    crate = ":runner",
     edition = "2018",
-    deps = [":cargo_build_script_runner"],
 )