`alias_attribute` used to define a "jump method", e.g.
`alias_attribute :foo, :bar` was pretty much a macro to generate

```ruby
def foo
  bar
end
```

This is convienient because it's easy, it doesn't impose an order
of declaration or anything like that.

But it's also much less efficient than a true `alias_method`.

It also used to cause cache size explosion which we fixed in
rails#52118, but making it behave
like Ruby's `alias_method`, by doing a real alias.

But this breaks some expectations (literally from the documentation):

```ruby
  attr_accessor :name
  attribute_method_suffix '_short?'
  define_attribute_methods :name

  alias_attribute :nickname, :name
```

Here we're not aliasing a generated method, but a user defined one.

A solution is to check if the method exist in the class itself,
and if it does, alias that user defined method instead of generating one.

It solves the issue at hand, but still is more restrictive than
the original implementation, as the user method has to be defined
before the `alias_attribute` call, but I think that's an OK limitation.

Another limitation is that user defined methods have to be defined
in the class itself. If they are defined in an ancestor the issue
remains.