Wrap qiskit.Aer and qiskit.IBMQ with lazy loading object (Qiskit/qisk…

…it#5619)

* Wrap qiskit.Aer and qiskit.IBMQ with lazy loading object

This commit migrates the qiskit.Aer and qiskit.IBMQ module attributes to
be lazy loading instances of lazy loading wrapper classes. The intent
here is to avoid importing from qiskit-aer or qiskit-ibmq-provider from
qiskit-terra, while this is safe while the packages share a shared
namespace we've been actively working to remove the use of namespace
packaging (see Qiskit/qiskit#5089, Qiskit/qiskit#4767, and Qiskit/qiskit#559) and the circular
dependency caused by re-exporting these attributes is blocking progress
on this. By using a lazy loading wrapper class we avoid an import type
circular dependency and opportunistically use qiskit-aer and/or
qiskit-ibmq-provider at runtime only if they're present after everything
is imported. This also may have some benefit for the overall import
performance of qiskit (being tracked in Qiskit/qiskit#5100) as it removes qiskit-aer
and qiskit-ibmq-provider from the import path unless they're being used.
Although the presence of qiskit.__qiskit_version__ might prevent any
performance improvements as it still imports all the elements to get
version information (and will be tackled in a separate PR).

Fixes Qiskit/qiskit#5532

* Fix lint

* Fix test lint

* DNM: test with ignis patch

* Revert "DNM: test with ignis patch"

This reverts commit ac9611c3ace30d101a70bda06e1987df14662182.

* Use ignis from source for tutorial job

* Update release note to be more clear

test/test_vqe.py

@@ -10,6 +10,8 @@
 # copyright notice, and modified files need to carry a notice indicating
 # that they have been altered from the originals.
 
+# pylint: disable=no-name-in-module,import-error
+
 """ Test VQE """
 
 import unittest
@@ -167,7 +169,7 @@ def test_with_aer_statevector(self):
         """Test VQE with Aer's statevector_simulator."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -190,7 +192,7 @@ def test_with_aer_qasm(self):
         """Test VQE with Aer's qasm_simulator."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -215,7 +217,7 @@ def test_with_aer_qasm_snapshot_mode(self):
         """Test the VQE using Aer's qasm_simulator snapshot mode."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -308,7 +310,7 @@ def test_vqe_expectation_select(self):
         """Test expectation selection with Aer's qasm_simulator."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return