Replace usage of google.api.core with google.api_core (#4221)

* Remove api.core packages from google.cloud.core, make google.cloud.core depend on api_core.

* s/google.api.core/google.api_core/g and nox updates

* Fixing core tests, addressing review feedback

* Fix bigquery

Author: Jon Wayne Parrott

google/api_core/future/polling.py

@@ -100,8 +100,8 @@ def result(self, timeout=None):
             google.protobuf.Message: The Operation's result.
 
         Raises:
-            google.gax.GaxError: If the operation errors or if the timeout is
-                reached before the operation completes.
+            google.api_core.GoogleAPICallError: If the operation errors or if
+                the timeout is reached before the operation completes.
         """
         self._blocking_poll(timeout=timeout)
 
@@ -120,7 +120,8 @@ def exception(self, timeout=None):
                 If None, wait indefinitely.
 
         Returns:
-            Optional[google.gax.GaxError]: The operation's error.
+            Optional[google.api_core.GoogleAPICallError]: The operation's
+                error.
         """
         self._blocking_poll()
         return self._exception

nox.py

@@ -42,7 +42,6 @@ def unit_tests(session, python_version):
     session.run(
         'py.test',
         '--quiet',
-        '--cov=google.cloud',
         '--cov=google.api_core',
         '--cov=tests.unit',
         '--cov-append',