Avoid relying on `typing.TYPE_CHECKING` to the maximum possible extent

[leofang]

During investigation of #454 and related doc rendering issues, it has come to my attention that the type checking detection logic does not work well with Sphinx (ex: sphinx-doc/sphinx#13137, sphinx-doc/sphinx#11225).

Independently, Ralf has pointed out that it makes code debugging harder (#463 (comment)).

The idea of having a if TYPE_CHECKING: check is to avoid unnecessary imports or circular imports. If such issues do not exist, we should avoid using it.