`cddl-derive` socket/plug extensions produce duplicate types

Socket/plug extensions are treated as separate types when they should ideally be merged into one enum.

Repro:
```cddl
$foo /= int
$foo /= tstr
```
Output:
```rust
pub type Foo = i64;
pub type Foo = String;
```
Error: ```the name `Foo` is defined multiple times```