feat: initial duplicate bevy version lint

[DaAlbrecht]

A first draft of a quick lint to check if multiple versions of bevy are defined. #15

todos:

• crate ui test
• better error message, perhaps use cargo metadata to get information about the crate versions.
• fix the span ( this should point to the crate causing the issue in the Cargo.toml)

error: Multiple versions of `bevy` found
  --> /Users/didi/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy-0.13.2/src/lib.rs:1:1
   |
1  | / #![allow(clippy::single_component_path_imports)]
2  | |
3  | | //! [![](https://bevyengine.org/assets/bevy_logo_d...
4  | | //!
...  |
51 | | #[allow(unused_imports)]
52 | | use bevy_dylib;
   | |_______________^
   |
   = note: `#[deny(bevy::duplicate_bevy_dependencies)]` on by default

[BD103]

So far this looks good! I am interested in getting a span to Cargo.toml / Cargo.lock, though I'm unsure if the compiler will know that file exists. Ping me when this is ready for a final review!

[BD103]

(Nit) Calling Symbol::intern() / sym!() is an expensive operation that should probably be cached. I solve this by storing the Symbol as a field in the lint pass in #184.

// Sketch of what you may do...
struct DuplicateBevyDependencies {
    bevy_symbol: Symbol,
}

impl Default for DuplicateBevyDependencies {
    fn default() -> Self {
        Self { bevy_symbol: sym!(bevy) }
    }
}

// This is different from `declare_lint_pass!`, since it does not actually define the lint pass structure.
impl_lint_pass! {
    DuplicateBevyDependencies => [DUPLICATE_BEVY_DEPENDENCIES.lint]
}