TransformerGenerator: support external functor types

[jeffijoe]

This PR adds support for generating transformer methods for functors that are external to the consumer. This means that consumers can now use [Functor] and [GenerateTransformer] on their own types and have methods generated that have the FxKit types on the outside.

Example:

using FxKit.CompilerServices;

namespace App;

[Functor]
public record Box<T>(T Item);

public static class Box
{
    [GenerateTransformer] // will have a transformer generated, and will be used by generated transformers.
    public static Box<U> Map<T, U>(this Box<T> source, Func<T, U> selector) =>
        new Box<U>(selector(source.Item));

    [GenerateTransformer] // will have a transformer generated
    public static T Unwrap(this Box<T> box) => box.Item;
}

This will now result in MapT and UnwrapT-variant being generated for every known functor type based on project references as well as assembly references.

var box = new Box<int>(2);
var outer = Task.FromResult(box);
Task<Box<string>> mapped = outer.MapT(x => x.ToString());
Task<string> unwrapped = mapped.UnwrapT();

Additionally, the generator has been restructured to properly take advantage of incremental generation. Before, the generator was run on virtually any change.