Add ability for pass sequence builder to locate next relationship based on only previous

[thinker227]

Add the ability for CompilerPassSequenceBuilder to be able to find the next pass in sequence based on the Previous property of another pass.

CompilerPassSequenceBuilder builder = new();
builder.Root = "a";
builder.AddPass("a");
builder.AddPass("b")
    .WithPrevious("a");

The above would succeed despite pass a not having specified a Next, because b specifies a as its Previous. This would obviously require lookup based on the Previous property of the currently processed pass, although this could be somewhat mitigated by constructing a dictionary with the Previous as the key. There would also have to be some mechanism to ensure that if multiple passes specify the same Previous then an exception would be thrown.