Separate `OrchardCore.Commerce` into more projects (OCC-81)

Is your feature request related to a problem? Please describe

At least create a OrchardCore.Commerce.Core, probably also a OrchardCore.Commerce.Prices. Organize the projects, so we can create modules without having to throw everything into OrchardCore.Commerce as an everything-bucket.

Describe the solution you'd like

OrchardCore.Commerce module should only contain the cross-cutting concerns, everything else should be moved to new lower level modules. Note that #149 is related, but it has other enhancements. The two can be done in any order.

Describe alternatives you've considered

Currently we have almost everything in that one module. We have already started moving away from that with OrchardCore.Commerce.ContentFields, but so far anything that involves existing content parts or services has to go into OrchardCore.Commerce which is not ideal.

Jira issue

Start by inspecting the OrchardCore.Commerce project structure and comparing it with the existing OrchardCore.Commerce.ContentFields module. Map the projects, content parts, and services that must move, then determine the dependencies between the proposed Core and Prices modules. Done means cross-cutting concerns remain in OrchardCore.Commerce and other functionality is separated into usable lower-level modules.