Generator - Add support for types with base type in another assembly

[TimLariviere]

Add a cache of loaded assemblies in the generator to support types with one or more base types in other assemblies.
This allows for scenario where we want to use a custom type that extends the base Xamarin.Forms types.

"assemblies": [
    "packages/neutral/Xamarin.Forms/lib/netstandard1.0/Xamarin.Forms.Core.dll",
    "src/Fabulous.Core/bin/Release/netstandard2.0/Fabulous.Core.dll",
],

    {
      // Extends and replaces Xamarin.Forms.Entry
      "name": "Fabulous.DynamicViews.CustomEntry",
      "modelName": "Entry",
      "members": [
        {
          "name": "PropertyThatOnlyExistsInCustomEntry",
          "defaultValue": "null"
        },
        {
          "name": "Text",
          "defaultValue": "null"
        },

Currently if Mono.Cecil tries to load the base types of a custom control (Fabulous.Core.dll) that are found in another assembly (Xamarin.Forms.Core.dll), Mono.Cecil throws an exception because it can't find Xamarin.Forms.Core

The need comes from this issue: #225

[TimLariviere]

Seems like Travis did not like my mistake when pushing the branch on fsprojects/Fabulous instead of my fork, and then deleting it.

[londospark]

Would it be worth me grabbing this fork for tonight's stream? I'm streaming in just over an hour and working with this would be really fun.

[TimLariviere]

@garethhubball Yes, this would be a good way to validate these changes against a real example. :)