Add things needed for rules_cc to build & test in the open

* Adding .gitignore
* Referencing protobuf, absl, and six in the WORKSPACE
* Adding six.BUILD

RELNOTES: None
PiperOrigin-RevId: 228309963

.gitignore

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+/bazel-*

WORKSPACE

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+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+http_archive(
+    name = "six_archive",
+    urls = [
+        "http://mirror.bazel.build/pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz",
+        "https://pypi.python.org/packages/source/s/six/six-1.10.0.tar.gz",
+    ],
+    sha256 = "105f8d68616f8248e24bf0e9372ef04d3cc10104f1980f54d57b2ce73a5ad56a",
+    strip_prefix = "six-1.10.0",
+    build_file = "@//third_party:six.BUILD",
+)
+
+bind(
+    name = "six",
+    actual = "@six_archive//:six",
+)
+
+http_archive(
+    name = "com_google_protobuf",
+    sha256 = "d6618d117698132dadf0f830b762315807dc424ba36ab9183f1f436008a2fdb6",
+    strip_prefix = "protobuf-3.6.1.2",
+    urls = ["https://github.com/google/protobuf/archive/v3.6.1.2.zip"],
+)
+
+http_archive(
+    name = "io_abseil_py",
+    sha256 = "74a2203a9b4681851f4f1dfc17f2832e0a16bae0369b288b18b431cea63f0ee9",
+    strip_prefix = "abseil-py-pypi-v0.6.1",
+    urls = ["https://github.com/abseil/abseil-py/archive/pypi-v0.6.1.zip"],
+)

third_party/com/github/bazelbuild/bazel/src/main/protobuf/BUILD

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+licenses(["notice"])  # Apache 2.0
+
+load("@com_google_protobuf//:protobuf.bzl", "py_proto_library")
+
+py_proto_library(
+    name = "crosstool_config_py_pb2",
+    srcs = ["crosstool_config.proto"],
+    visibility = [
+        "//tools/migration:__pkg__",
+    ],
+)

third_party/six.BUILD

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+# Description:
+#   Six provides simple utilities for wrapping over differences between Python 2
+#   and Python 3.
+
+licenses(["notice"])  # MIT
+
+exports_files(["LICENSE"])
+
+py_library(
+    name = "six",
+    srcs = ["six.py"],
+    srcs_version = "PY2AND3",
+    visibility = ["//visibility:public"],
+)