Associations don't respect namespaces when defining PrimaryKeyType

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Given:

```crystal
module MyApp
class Post < BaseModel
table do
belongs_to user : User
end
end
end
```

If I try to define a SaveOperation for this:

```crystal
class SavePost < MyApp::Post::SaveOperation
#... blah
end
```

I'll get the error:

```
web | > 32 | user_id : User::PrimaryKeyType | Nothing = Nothing.new,
web | ^
web | Error: undefined constant User::PrimaryKeyType
```

The current work-around is if your model is namespaced, then the associations must also be defined with the namespace.

```crystal
module MyApp
class Post < BaseModel
table do
belongs_to user : MyApp::User
end
end
end
```

*EDIT*: ok, so doing that "work-around" actually doesn't quite work. It does let me make my save operation, but my queries are borked with `undefined method where_posts for MyApp::User`. Without looking, I'm assuming adding the namespace changes this...

Start by reproducing the namespaced MyApp::Post and MyApp::User association examples, then trace how SaveOperation generates User::PrimaryKeyType and how queries generate where_posts. Done means namespaced associations compile without manual qualification and the resulting queries work correctly.

Written by the indexing model from the issue text.