refactor, put `PostgreSQLColumn` into `column.rb`.

We have `connection_adapters/column.rb` so it's easier to remember
that the column in in a separate file.
PostgreSQL determine `Column#type` through corresponding OID. #7814

I ran the whole test suite and compared the old to the new types.
Following is the list of types that did change with this patch:

```
DIFFERENT TYPE FOR mood: NEW: enum, BEFORE:
DIFFERENT TYPE FOR floatrange: NEW: floatrange, BEFORE: float
```

The `floatrange` is a custom type. The old type `float` was simply a coincidence
form the name `floatrange` and our type-guessing.