…rds for documentation of estimator types (sktime#6998)

Fixes sktime#6970 by a redesign of the
object scitype register that decouples the `registry` module from the
rest of `sktime`, by removing all outside imports on module level
(except from `sktime.base` and `utils`).

As a side effect, this also adds one record class per object type, which
can be used as a tagged metadata record and later as a basis or
documenting the individual object types in `sktime`.

The refactor proceeds as follows:

* the base class register is replaced by data record classes similar to
the `_tags` module
* the imports of base classes are isolated in class methods of those
records, `get_base_class`, which returns the base class corresponding to
the scitype
* exports of objects involving tags, in particular the classes, are
replaced by functions that produce the object, further isolating the
import to places where it is needed, e.g., `get_base_class_list`

To make the changes deprecation safe, imports of coupled objects are
intercepted, and replaced by calls on demand, i.e., whenever an external
call carries out an import.

Further, imports from outside `registry` but inside `sktime` (all from
the test framework) are also replaced with the new functions. This lead
to some dead functions and objects, which were removed, which further
lead to unused imports, which also were removed.