Rename 'pyiree' to 'iree' (#5145)

Renames all non-BUILD references to `pyiree` (`BUILD` files are changed in 
an internal patch). Simplifies the imports in some places to be more consistent.
Also deletes some old test files that use the old API and enables the tests for 
`iree.tf.support`. 
diff --git a/integrations/tensorflow/e2e/dynamic_mlp_test.py b/integrations/tensorflow/e2e/dynamic_mlp_test.py
index 4a531e6..e1ef21a 100644
--- a/integrations/tensorflow/e2e/dynamic_mlp_test.py
+++ b/integrations/tensorflow/e2e/dynamic_mlp_test.py
@@ -14,9 +14,9 @@
 # limitations under the License.
 
 from absl import app
+from iree.tf.support import tf_test_utils
+from iree.tf.support import tf_utils
 import numpy as np
-from pyiree.tf.support import tf_test_utils
-from pyiree.tf.support import tf_utils
 import tensorflow.compat.v2 as tf
 
 HIDDEN_1_DIM = 256