Mostly to fix the TODO, doesn't have much semantic meaning.

Also fixes a related typo in import inside `TYPE_CHECKING` block.

It's not different from the `tmp_path_factory` already requested via
fixture.

pyright (somewhat annoyingly IMO) infers unannotated function types,
unlike mypy which treats them as `Any`. In these 2 cases it causes some
follow on type errors, so add explicit `Any`s to prevent pyright from
inferring some big union.

For some reason pyright infers the type of `essential_plugins` and
`default_plugins` as `tuple[Literal[...], ...]` even if they are
explicitly annotated as `tuple[str, ...]`. This then causes an error in
the `builtin_plugins.add` calls.

Replace with one-shot initialization appeases pyright and is nicer
regardless.

This is the expected case for casts -- the correct type is ensured
dynamically, just too complicated for a type checker to understand.

Pyright complains:

    Type annotation for "self" parameter of "__init__" method cannot
    contain class-scoped type variables (reportInvalidTypeVarUse)

and I figure it's not needed since it's already the type of `self`.

Pyright doesn't handle this.

There is not much reason to use `TypeGuard` on the implementation
function of an `@overload`ed function, since only the types of the
overloads are used.

This fixes a pyright "Overloaded implementation is not consistent with
signature of overload" error.

For manual testing, as we are not yet passing.