Remove useless module.hot checks

Summary:
Since we always create `module.hot` objects, the `module.hot` checks were unnecessary. They give a false impression that we're checking for a Hot Reloading mode. However, they're just Flow refinements and always exist in DEV. I made that explicit by throwing early.

Similarly, I removed a `module.hot` check inside `setupReactRefresh`, as it is always truish in DEV.

Finally, I'm adding a new mechanism as an escape hatch. It lets you do:

```
if (__DEV__) {
  require.Refresh.forceFullRefresh = true;
}
```

in your entry point and opt into full refreshes on every edit. This sounds similar to "Reload-on-Save". That is because in the next diff, I plan to remove "Reload-on-Save" from user-visible options (but it'll stay for automated workflows).

So this workaround is intended for people who for one reason or another don't want to opt into Hot Reloading as an alternative. We'll need to talk to them and find out why.

Reviewed By: rickhanlonii

Differential Revision: D15958475

fbshipit-source-id: 674187ddf86a4e286dfae28f4182555a8b5d7396