Beta API clean-ups

(1) Renamed the "beta" module "implementations" because it hasn't been
monolithic since "interfaces" was factored out of it a few changes
back.

(2) Moved ChannelConnectivity from grpc.beta.beta to
grpc.beta.interfaces since it is constants that don't depend on the
beta implementation.

(3) Moved the Server interface definition from grpc.beta.beta to
grpc.beta.interfaces since it is an interface.

(4) Dropped the "create_" prefix from "create_<...>_channel" functions
to better match the other creation functions throughout the codebase.
diff --git a/src/compiler/python_generator.cc b/src/compiler/python_generator.cc
index fe2b9fa..83133f2 100644
--- a/src/compiler/python_generator.cc
+++ b/src/compiler/python_generator.cc
@@ -339,7 +339,7 @@
     }
     out->Print("}\n");
     out->Print(
-        "return implementations.server("
+        "return early_adopter_implementations.server("
         "\"$PackageQualifiedServiceName$\","
         " method_service_descriptions, port, private_key=private_key,"
         " certificate_chain=certificate_chain)\n",
@@ -422,7 +422,7 @@
     }
     out->Print("}\n");
     out->Print(
-        "return implementations.stub("
+        "return early_adopter_implementations.stub("
         "\"$PackageQualifiedServiceName$\","
         " method_invocation_descriptions, host, port,"
         " metadata_transformer=metadata_transformer, secure=secure,"
@@ -586,13 +586,13 @@
                  "Constructor", name_and_implementation_constructor->second);
     }
     out->Print("}\n");
-    out->Print("server_options = beta.server_options("
+    out->Print("server_options = beta_implementations.server_options("
                "request_deserializers=request_deserializers, "
                "response_serializers=response_serializers, "
                "thread_pool=pool, thread_pool_size=pool_size, "
                "default_timeout=default_timeout, "
                "maximum_timeout=maximum_timeout)\n");
-    out->Print("return beta.server(method_implementations, "
+    out->Print("return beta_implementations.server(method_implementations, "
                "options=server_options)\n");
   }
   return true;
@@ -685,13 +685,13 @@
                  "Cardinality", name_and_cardinality->second);
     }
     out->Print("}\n");
-    out->Print("stub_options = beta.stub_options("
+    out->Print("stub_options = beta_implementations.stub_options("
                "host=host, metadata_transformer=metadata_transformer, "
                "request_serializers=request_serializers, "
                "response_deserializers=response_deserializers, "
                "thread_pool=pool, thread_pool_size=pool_size)\n");
     out->Print(
-        "return beta.dynamic_stub(channel, \'$PackageQualifiedServiceName$\', "
+        "return beta_implementations.dynamic_stub(channel, \'$PackageQualifiedServiceName$\', "
         "cardinalities, options=stub_options)\n",
         "PackageQualifiedServiceName", package_qualified_service_name);
   }
@@ -701,9 +701,9 @@
 bool PrintPreamble(const FileDescriptor* file,
                    const GeneratorConfiguration& config, Printer* out) {
   out->Print("import abc\n");
-  out->Print("from $Package$ import beta\n",
+  out->Print("from $Package$ import implementations as beta_implementations\n",
              "Package", config.beta_package_root);
-  out->Print("from $Package$ import implementations\n",
+  out->Print("from $Package$ import implementations as early_adopter_implementations\n",
              "Package", config.early_adopter_package_root);
   out->Print("from grpc.framework.alpha import utilities as alpha_utilities\n");
   out->Print("from grpc.framework.common import cardinality\n");