Ok to add CollectGenericUnion
but around GetMiniallyQualifiedClassName line, it has some removed code.
I don't know removed code cause what sideeffect.
Can you explain more info?
I worry about the possibility of breaking the existing code.

The template string was used to generate the formatter classes with a generic type definition.
I found a bug in that approach, e.g.

[MessagePackObject]
public class TreeNode<T> 
{
    [Key(0)]
    public TreeNode<T> Parent { get; set; }   
}

[MessagePackObject]
public class Leaf : TreeNode<Leaf> { }

[MessagePackObject]
public class Leaf2 : TreeNode<Leaf2> { }

When you use mpc on this code it will generate the following:

public sealed class TreeNodeFormatter<T> : IMessagePackFormatter<TreeNode<Leaf>>
{  ...  }
public sealed class TreeNodeFormatter<T> : IMessagePackFormatter<TreeNode<Leaf2>>
{  ...  }

There will be two formatters created with the name TreeNodeFormatter<T> causing a compile error. The new GetMiniallyQualifiedClassName fixes the issue by explicitly stating the type names in the name.

public sealed class TreeNode_Leaf_Formatter : IMessagePackFormatter<TreeNode<Leaf>>
{  ...  }
public sealed class TreeNode_Leaf2_Formatter : IMessagePackFormatter<TreeNode<Leaf2>>
{  ...  }

@neuecc I am not 100% that this will not break anything, what do you think?