Turn `//python/private` into a package. (#555)

python/BUILD

diff --git a/python/BUILD b/python/BUILD
index 88ee4dc..972bf9e 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -36,6 +36,7 @@
         "//python/constraints:distribution",
         "//python/legacy_pip_import:distribution",
         "//python/runfiles:distribution",
+        "//python/private:distribution",
     ],
     visibility = ["//:__pkg__"],
 )
@@ -48,8 +49,8 @@
         "defs.bzl",
         "packaging.bzl",
         "pip.bzl",
-        "private/reexports.bzl",
         "whl.bzl",
+        "//python/private:bzl",
     ],
     visibility = ["//:__pkg__"],
 )
@@ -61,14 +62,6 @@
     "python.bzl",  # Deprecated, please use defs.bzl
 ])
 
-# Needed to define bzl_library targets for docgen. (We don't define the
-# bzl_library target here because it'd give our users a transitive dependency
-# on Skylib.)
-exports_files(
-    ["private/reexports.bzl"],
-    visibility = ["//docs:__pkg__"],
-)
-
 # This target can be used to inspect the current Python major version. To use,
 # put it in the `flag_values` attribute of a `config_setting` and test it
 # against the values "PY2" or "PY3". It will always match one or the other.