bevy_input: allow use without bevy_reflect

[torsteingrindvik]

Objective

Allow use of bevy_input types without needing bevy_reflect.

Solution

Make bevy_reflect within bevy_input optional. It's compiled in by default. Turn on reflect in dependencies as well when this feature is on.

Testing

• Did you test these changes? If so, how?

I did a cargo hack -p bevy_input --each-feature build.