Move IBMQ provider 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 aer perspective by moving
qiskit.providers.ibmq to it's own package and namespace
'qiskit_ibmq_provider'.

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 the IBMQ provider.

Details and comments

Depends-On: Qiskit/qiskit#4767
Fixes Qiskit/qiskit#559

[mtreinish]

Besides all the issues and CI here, this is failing unittests on aer because of a weird circular import error. While I think we could work through most of the ibmq provider failures pretty quickly I have no idea on the source of the aer failure as of yet. Since this will likely take me some time to debug I'm setting this as on hold as a revert is proposed at Qiskit/qiskit#5006