Reduce dependencies on `bevy_render` by preferring `bevy_mesh` imports
over `bevy_render` re-exports.
```diff
- use bevy_render::mesh::Mesh;
+ use bevy_mesh::Mesh;
```
This is intended to help with #18423 (render crate restructure). Affects
`bevy_gltf`, `bevy_animation` and `bevy_picking`.
As part of #18423, I'm assuming there'll be a push to make crates less
dependent on the big render crates. This PR seemed like a small and safe
step along that path - it only changes imports and makes the `bevy_mesh`
crate dependency explicit in `Cargo.toml`. Any remaining dependencies on
`bevy_render` are true dependencies.
```
cargo run --example testbed_3d
cargo run --example mesh_picking
```
# Objective
Refactor `bevy_gltf`, the criteria for the split is kind of arbitrary
but at least it is not a 2.6k line file.
## Solution
Move methods and structs found in `bevy_gltf/loader.rs` into multiple
new modules.
## Testing
`cargo run -p ci`
