Currently GeneratedAttributeMethods is a module builder class, an
instance of which is included in every AR class. OTOH,
GeneratedAssociatedMethods is assigned to a constant under the model
namespace. This is inconsistent and looks strange in the list of
ancestors.

There is no particular reason *not* to assign a constant for this (very
important) module under the model namespace, so that's what this commit
does.

Previous to this change, ancestors for an AR class looked like this:

```
=> [User (call 'User.connection' to establish a connection),
 User::GeneratedAssociationMethods,
 #<ActiveRecord::AttributeMethods::GeneratedAttributeMethods:0x000055ace0f05b08>,
 ApplicationRecord(abstract),
 ApplicationRecord::GeneratedAssociationMethods,
 #<ActiveRecord::AttributeMethods::GeneratedAttributeMethods:0x000055ace093c460>,
 ActiveRecord::Base,
 ...
```

With this change, they look like this:

```
=> [User (call 'User.connection' to establish a connection),
 User::GeneratedAssociationMethods,
 User::GeneratedAttributeMethods,
 ApplicationRecord(abstract),
 ApplicationRecord::GeneratedAssociationMethods,
 ApplicationRecord::GeneratedAttributeMethods,
 ActiveRecord::Base,
 ...
```

The previously named `GeneratedAttributeMethods` module builder class is
renamed `GeneratedAttributeMethodsBuilder` to emphasize that this is not
a module but a class.