Move Ignis to its own package

[mtreinish]

Summary

Namespace packages are constant source of problems for users. The python
packaging ecosystem around splitting packages across namespaces is
fragile at the best of times and can often leave a you with an
environment that isn't recoverable (especially when mixing install
methods). There is also a performance hit whenever there is a piece of
the namespace we allow external packages to extend since it requires
doing a full python path search which can be slow depending on the
backing I/O and the number of paths in sys.path for an environment. This
commit addresses the piece from the ignis perspective by moving
qiskit.ignis to it's own package and namespace 'qiskit_ignis'.

This will be coupled with a change in terra that removes the arbitrary
namespace hook points and hard coding the element namespace maps via a
custom import loader at the root of the namespace. This has 2 advantages
it removes the use of namespace packages so the fragility and
performance impact are fixed since every element will be renamed to use
'qiskit_' instead of 'qiskit.', but it also makes it explicit where we
extend the namespace. The previous method allowed any package to extend
qiskit.* and qiskit.providers.* with whatever they wanted. This commit
will need to be coordinated with the terra change to ensure we don't
block development, because while it's not breaking for end users as a
coordinated code release piecewise it's a breaking change for each
element, including Ignis.

Details and comments

Depends-On: Qiskit/qiskit#4767

Fixes Qiskit/qiskit#559

[mtreinish]

Setting this as on hold because other elements are having issues with this in their CI, so a revert is proposed at Qiskit/qiskit#5006

[mtreinish]

With the creation of the qiskit-experiments repo this isn't needed anymore because that project will subsume this when it's ready and already exists in a separate namespace.