Support loading ASP.NET Core into non-default load contexts

### Describe the solution you'd like

Users creating a module system that uses ASP.NET Core will run into problems if they are using a non-default `AssemblyLoadContext`. The reason is that we have various features that operate outside of a load context, usually by using `LoadFile` instead of one of the more context-aware methods.

Known features that don't support ALCs:

- Loading related application parts (views/pages)
- (possibly more)

### Additional context

While we haven't totally ignored this as an area, it's not something we document and test. At a minimum completing this would would require identifying a few scenarios that are good as samples and documenting them. We have not done a full effort to ensure that ever part of ASP.NET Core will work in this mode.

Original source: filing a feature request based on a support ticket for an unsupported scenario.