build: add means of linking with C libs in Py packages (#1668)

Extend the Python repository_rule used to create external repositories, adding targets for C-language binary libraries shipped inside Python packages; e.g., that shipped in package tensorflow-gpu.

These targets are to be used as dependencies by C-language targets.

Note: when debugging the build, it can be helpful to examine the repository directory and BUILD file this repository_rule generates in the bazel cache.

Begin using a python/ directory at the root of the project for code that is specific to Python.

Upgrade to the latest version of rules_python first. Note that the unit test to keep requirements.in and requirements.txt is disabled (specifically with 
ec6bdc5d4443285d28a44076f06c203d9582e4a1)

Add a unit test for this feature.

BUG=see description

python/tests/BUILD

@@ -0,0 +1,22 @@
+cc_test(
+    name = "cc_deps_link_test",
+    size = "small",
+    srcs = [
+        "cc_deps_link_test.cc",
+    ],
+    copts = [
+        "-std=c++14",
+        # Compiling against upstream Tensorflow headers requires -std=c++14
+        # due (at least) to its dependency on Eigen, which raises the error
+        # "This compiler appears to be too told to be supported by Eigen"
+        # via the preprocessor, as well as several compilation errors.
+        #
+        # TODO(b/271210933): Harmonize this with a global solution
+        # throughout the repository. Presumably this same issue will be
+        # encountered by other code compiling against upstream Tensorflow,
+        # such as custom operator implementations.
+    ],
+    deps = [
+        "@tensorflow_cc_deps//:cc_library",
+    ],
+)

python/tests/cc_deps_link_test.cc

@@ -0,0 +1,12 @@
+// A simple program to test the py_pkg_cc_deps repository rule by building and
+// linking against the Tensorflow library shipping in the Tensorflow Python
+// package.
+
+#include <tensorflow/core/util/util.h>
+
+int main(int argc, char* argv[]) {
+  const char* ptr = "test";
+  const size_t n = 4;
+  tensorflow::PrintMemory(ptr, n);
+  return 0;
+}