Deprioritize imports within a `if TYPE_CHECKING:` when processing import cycles

If an import is ignored at runtime (due to being in an `if TYPE_CHECKING:` or `if MYPY:` block), it should always be considered to be of the lowest priority when sorting import cycles. This is important, in particular, since the imports could well be of form `from m import ...`, which are given the highest priority right now.

There is more discussion in #481.
