chore: Adjust gapic namespace to google.cloud.bigquery_datatransfer (#61)

* chore: use google.cloud.bigquery_datatransfer namespace

Using google.cloud.bigquery.datatransfer turned out to be too
problematic to use with the google.cloud.bigquery library
(namespace clashes).

* Adjust code to renamed namespace

* Update UPGRADING guide with new (old) namespace

Author: plamut

bigquery-datatransfer/snippets/quickstart.py

@@ -17,9 +17,9 @@
 
 def run_quickstart(project="my-project"):
     # [START bigquerydatatransfer_quickstart]
-    from google.cloud.bigquery import datatransfer
+    from google.cloud import bigquery_datatransfer
 
-    client = datatransfer.DataTransferServiceClient()
+    client = bigquery_datatransfer.DataTransferServiceClient()
 
     # TODO: Update to your project ID.
     # project = "my-project"