[bazel] Fix python BUILD rules

* Add missing cython sources
* Make grpcio a library as it is not a binary

src/python/grpcio/grpc/BUILD.bazel

@@ -2,7 +2,7 @@ load("@grpc_python_dependencies//:requirements.bzl", "requirement")
 
 package(default_visibility = ["//visibility:public"])
 
-py_binary(
+py_library(
     name = "grpcio",
     srcs = ["__init__.py"],
     deps = [
@@ -22,7 +22,6 @@ py_binary(
     data = [
         "//:grpc",
     ],
-    main = "__init__.py",
     imports = ["../",],
 )
 

src/python/grpcio/grpc/_cython/BUILD.bazel

@@ -8,6 +8,7 @@ pyx_library(
         "__init__.py",
         "cygrpc.pxd",
         "cygrpc.pyx",
+        "_cygrpc/_hooks.pyx.pxi",
         "_cygrpc/grpc_string.pyx.pxi",
         "_cygrpc/arguments.pyx.pxi",
         "_cygrpc/call.pyx.pxi",
@@ -15,6 +16,7 @@ pyx_library(
         "_cygrpc/credentials.pyx.pxi",
         "_cygrpc/completion_queue.pyx.pxi",
         "_cygrpc/event.pyx.pxi",
+        "_cygrpc/fork_posix.pyx.pxi",
         "_cygrpc/metadata.pyx.pxi",
         "_cygrpc/operation.pyx.pxi",
         "_cygrpc/records.pyx.pxi",
@@ -24,12 +26,14 @@ pyx_library(
         "_cygrpc/time.pyx.pxi",
         "_cygrpc/grpc_gevent.pyx.pxi",
         "_cygrpc/grpc.pxi",
+        "_cygrpc/_hooks.pxd.pxi",
         "_cygrpc/arguments.pxd.pxi",
         "_cygrpc/call.pxd.pxi",
         "_cygrpc/channel.pxd.pxi",
         "_cygrpc/credentials.pxd.pxi",
         "_cygrpc/completion_queue.pxd.pxi",
         "_cygrpc/event.pxd.pxi",
+        "_cygrpc/fork_posix.pxd.pxi",
         "_cygrpc/metadata.pxd.pxi",
         "_cygrpc/operation.pxd.pxi",
         "_cygrpc/records.pxd.pxi",