Rename canonical workspace name to "@rules_python" (#212)

* Change official workspace name to @rules_python

This includes regenerating the par files to use the new name. Neat trick: Since
the par file regeneration depends on the previous par files, I had to bootstrap
this change by temporarily editing the WORKSPACE to include:

local_repository(
    name = "io_bazel_rules_python",
    path = ".",
)

* Add a nice error message to help with the workspace name migration

This hooks into pip_repositories(), which users are *supposed* to be calling in
their WORKSPACE files, to emit a nice fail() message alerting them that they
need to update their repo definition.

Without this change (and even with it, for users who do not call
`pip_repositories()`), users will instead see a confusing cyclic dependency
error.

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 296055f..abf1a47 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-workspace(name = "io_bazel_rules_python")
+workspace(name = "rules_python")
 
 load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")