Add exports for core Python logic that's bundled with Bazel (#202)

* Introduce defs.bzl as the official home of the core Python rules

The "core" Python rules are the rules that traditionally have been bundled with
Bazel. This includes native rules like `py_binary`, and Starlark-defined rules
under `@bazel_tools` like `py_runtime_pair`. These should all live in
or around `@rules_python//python:defs.bzl`.

Currently we re-export the native rules here, with a magic tag to allow them to
survive the flag flip for `--incompatible_load_python_rules_from_bzl`. When
native rules are ported to Starlark their definitions will live here.

* Add re-exports for Starlark-defined symbols

This adds export definitions for built-in symbols like `PyInfo` and
`@bazel_tools`-defined symbols like py_runtime_pair.

* Vendor in runfiles library

This vendors in the @bazel_tools//tools/python/runfiles target as
//python/runfiles. See comment in the BUILD file for why we couldn't re-export
the bundled implementation.

* Fix README to prefer defs.bzl over python.bzl

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index f222f39..f777f79 100644
--- a/README.md
+++ b/README.md
@@ -46,7 +46,7 @@ Then in your `BUILD` files load the python rules with:
 
 ``` python
 load(
-  "@rules_python//python:python.bzl",
+  "@rules_python//python:defs.bzl",
   "py_binary", "py_library", "py_test",
 )