Fix unsound lifetime annotation on Query::get_component (#2964)

#2605 changed the lifetime annotations on `get_component` introducing unsoundness as you could keep the returned borrow even after using the query.

Example unsoundness:
```rust
use bevy::prelude::*;

fn main() {
    App::new()
        .add_startup_system(startup)
        .add_system(unsound)
        .run();
}

#[derive(Debug, Component, PartialEq, Eq)]
struct Foo(Vec<u32>);

fn startup(mut c: Commands) {
    let e = c.spawn().insert(Foo(vec![10])).id();
    c.insert_resource(e);
}

fn unsound(mut q: Query<&mut Foo>, res: Res<Entity>) {
    let foo = q.get_component::<Foo>(*res).unwrap();
    let mut foo2 = q.iter_mut().next().unwrap();

    let first_elem = &foo.0[0];
    for _ in 0..16 {
        foo2.0.push(12);
    }
    dbg!(*first_elem);
}
```
output:
`[src/main.rs:26] *first_elem = 0`

crates/bevy_ecs/src/system/query.rs

@@ -699,10 +699,7 @@ where
     /// # print_selected_character_name_system.system();
     /// ```
     #[inline]
-    pub fn get_component<T: Component>(
+    pub fn get_component<T: Component>(&self, entity: Entity) -> Result<&T, QueryComponentError> {
-        &self,
-        entity: Entity,
-    ) -> Result<&'w T, QueryComponentError> {
         let world = self.world;
         let entity_ref = world
             .get_entity(entity)