Split bevy_hierarchy out from bevy_transform (#4168)

# Objective

- Hierarchy tools are not just used for `Transform`: they are also used for scenes.
- In the future there's interest in using them for other features, such as visiibility inheritance.
- The fact that these tools are found in `bevy_transform` causes a great deal of user and developer confusion
- Fixes #2758.

## Solution

- Split `bevy_transform` into two!
- Make everything work again.

Note that this is a very tightly scoped PR: I *know* there are code quality and docs issues that existed in bevy_transform that I've just moved around. We should fix those in a seperate PR and try to merge this ASAP to reduce the bitrot involved in splitting an entire crate.

## Frustrations

The API around `GlobalTransform` is a mess: we have massive code and docs duplication, no link between the two types and no clear way to extend this to other forms of inheritance.

In the medium-term, I feel pretty strongly that `GlobalTransform` should be replaced by something like `Inherited<Transform>`, which lives in `bevy_hierarchy`:

- avoids code duplication
- makes the inheritance pattern extensible
- links the types at the type-level
- allows us to remove all references to inheritance from `bevy_transform`, making it more useful as a standalone crate and cleaning up its docs

## Additional context

- double-blessed by @cart in bevyengine/bevy#4141 (comment) and bevyengine/bevy#2758 (comment)
- preparation for more advanced / cleaner hierarchy tools: go read bevyengine/rfcs#53 !
- originally attempted by @finegeometer in #2789. It was a great idea, just needed more discussion!

Co-authored-by: Carter Anderson <[email protected]>

Author: alice-i-cecile

Cargo.toml

@@ -14,6 +14,7 @@ bevy_app = { path = "../bevy_app", version = "0.6.0" }
 bevy_asset = { path = "../bevy_asset", version = "0.6.0" }
 bevy_core = { path = "../bevy_core", version = "0.6.0" }
 bevy_ecs = { path = "../bevy_ecs", version = "0.6.0" }
+bevy_hierarchy = { path = "../bevy_hierarchy", version = "0.6.0" }
 bevy_pbr = { path = "../bevy_pbr", version = "0.6.0" }
 bevy_reflect = { path = "../bevy_reflect", version = "0.6.0", features = ["bevy"] }
 bevy_render = { path = "../bevy_render", version = "0.6.0" }

src/loader.rs

@@ -4,6 +4,7 @@ use bevy_asset::{
 };
 use bevy_core::Name;
 use bevy_ecs::world::World;
+use bevy_hierarchy::{BuildWorldChildren, WorldChildBuilder};
 use bevy_log::warn;
 use bevy_math::{Mat4, Vec3};
 use bevy_pbr::{
@@ -24,11 +25,8 @@ use bevy_render::{
     view::VisibleEntities,
 };
 use bevy_scene::Scene;
-use bevy_transform::{
-    hierarchy::{BuildWorldChildren, WorldChildBuilder},
-    prelude::Transform,
-    TransformBundle,
-};
+use bevy_transform::{components::Transform, TransformBundle};
+
 use bevy_utils::{HashMap, HashSet};
 use gltf::{
     mesh::Mode,