Refactor EntryPointDriver to not compile against a specific simulator

[swernli]

Currently the EntryPointDriver is compiled against the default QuantumSimulator in a way that makes it necessary to create a new entry point driver project for each variant of the quantum simulator (like those built with different target packages). The entry point infrastructure already supports dynamically choosing an alternate simulator at runtime, so instead the explicit references to the default could be removed and the infrastructure reworked slightly to treat the built-in simulator the same was as a custom simulator, thus removing the linking between the two dlls and allowing any simulator variant to be used with the single entry point driver.

See #249 for the original context of this ask.

[filipw]

yes please 👏

[swernli]

After finishing the infrastructure for #249, we now compile the simulators without a direct dependency on a specific target package; that is to say, the Microsoft.Quantum.Simulators package no longer depends on Microsoft.Quantum.QSharp.Core nor does it assume the presence of a specific Q# intrinsic gate set. That should allow any simulator using any gate set to be built along with the entry point or loaded dynamically without requiring that simulator to use the QSharp.Core target package.