Bazel: add independent bazel directory

bazel/BUILD

+package(
+    default_visibility = ["//visibility:public"],
+)

bazel/common.bzl

+# Sanitize a dependency so that it works correctly from code that includes
+# TensorFlow as a submodule.
+def clean_dep(dep):
+    return str(Label(dep))
+
+# Ref: bazel-skylib@lib/paths.bzl
+def basename(p):
+    """Returns the basename (i.e., the file portion) of a path.
+    Note that if `p` ends with a slash, this function returns an empty string.
+    This matches the behavior of Python's `os.path.basename`, but differs from
+    the Unix `basename` command (which would return the path segment preceding
+    the final slash).
+    Args:
+      p: The path whose basename should be returned.
+    Returns:
+      The basename of the path, which includes the extension.
+    """
+    return p.rpartition("/")[-1]
+
+def dirname(p):
+    """Returns the dirname of a path.
+    The dirname is the portion of `p` up to but not including the file portion
+    (i.e., the basename). Any slashes immediately preceding the basename are not
+    included, unless omitting them would make the dirname empty.
+    Args:
+      p: The path whose dirname should be returned.
+    Returns:
+      The dirname of the path.
+    """
+    prefix, sep, _ = p.rpartition("/")
+    if not prefix:
+        return sep
+    else:
+        # If there are multiple consecutive slashes, strip them all out as Python's
+        # os.path.dirname does.
+        return prefix.rstrip("/")