Ensure all core Python rules are loaded from defs.bzl (bazel-contrib#219)

This migrates rules_python itself for bazelbuild/bazel#9006. This entails
adding or updating load() statements to refer to //python:defs.bzl (not
//python:python.bzl), and updating whl.py so the generated repos refer to
@rules_python. Also updated the dependency on subpar for compatibility with
the flag. Par files are regenerated.

Author: brandjon

WORKSPACE

@@ -63,7 +63,7 @@ _piptool_install()
 git_repository(
     name = "subpar",
     remote = "https://github.com/google/subpar",
-    tag = "2.0.0",
+    commit = "2917d275ee27d7935f7809413c74eddf71a46e5c",  # 2019-08-02
 )
 
 ###################################

examples/boto/BUILD

@@ -16,7 +16,7 @@ package(default_visibility = ["//visibility:public"])
 licenses(["notice"])  # Apache 2.0
 
 load("@examples_boto//:requirements.bzl", "requirement")
-load("//python:python.bzl", "py_test")
+load("//python:defs.bzl", "py_test")
 
 py_test(
     name = "boto_test",

examples/extras/BUILD

@@ -16,7 +16,7 @@ package(default_visibility = ["//visibility:public"])
 licenses(["notice"])  # Apache 2.0
 
 load("@examples_extras//:requirements.bzl", "requirement")
-load("//python:python.bzl", "py_test")
+load("//python:defs.bzl", "py_test")
 
 py_test(
     name = "extras_test",

examples/helloworld/BUILD

@@ -16,7 +16,7 @@ package(default_visibility = ["//visibility:public"])
 licenses(["notice"])  # Apache 2.0
 
 load("@examples_helloworld//:requirements.bzl", "requirement")
-load("//python:python.bzl", "py_library", "py_test")
+load("//python:defs.bzl", "py_library", "py_test")
 
 py_library(
     name = "helloworld",

examples/version/BUILD

@@ -16,7 +16,7 @@ package(default_visibility = ["//visibility:public"])
 licenses(["notice"])  # Apache 2.0
 
 load("@examples_version//:requirements.bzl", "requirement")
-load("//python:python.bzl", "py_test")
+load("//python:defs.bzl", "py_test")
 
 py_test(
     name = "version_test",

experimental/examples/wheel/BUILD

@@ -15,7 +15,7 @@ package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
 
-load("//python:python.bzl", "py_library", "py_test")
+load("//python:defs.bzl", "py_library", "py_test")
 load("//experimental/python:wheel.bzl", "py_package", "py_wheel")
 
 py_library(

experimental/examples/wheel/lib/BUILD

@@ -15,7 +15,7 @@ package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
 
-load("//python:python.bzl", "py_library")
+load("//python:defs.bzl", "py_library")
 
 py_library(
     name = "simple_module",

experimental/rules_python/BUILD

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-load("//python:python.bzl", "py_binary")
+load("//python:defs.bzl", "py_binary")
 
 py_binary(
     name = "wheelmaker",

rules_python/BUILD

@@ -15,7 +15,8 @@ package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
 
-load("//python:python.bzl", "py_binary", "py_library", "py_test")
+load("//python:defs.bzl", "py_library", "py_test")
+load("@subpar//:subpar.bzl", "par_binary")
 load("@piptool_deps//:requirements.bzl", "requirement")
 
 py_library(
@@ -42,8 +43,6 @@ py_test(
     ],
 )
 
-load("@subpar//:subpar.bzl", "par_binary")
-
 par_binary(
     name = "piptool",
     srcs = ["piptool.py"],

rules_python/whl.py

@@ -142,6 +142,7 @@ def main():
     f.write("""
 package(default_visibility = ["//visibility:public"])
 
+load("@rules_python//python:defs.bzl", "py_library")
 load("{requirements}", "requirement")
 
 py_library(