Fix loose mode fallback resolution

[Freakazo]

Summary

• match the Yarn reference PnP runtime by resolving loose-mode fallbacks from the top-level locator before the deduplicated fallback pool
• avoid applying fallback resolution when the issuer is the top-level package itself
• preserve undeclared and unfulfilled-peer states while evaluating fallbacks

Closes #111

[Freakazo]

I think this could be done cleaner with

enum FallbackResolution {
    Missing,
    Unbound,
    Resolved(PackageDependency),
}

and

match resolve_via_fallback(manifest, &ident) {
    FallbackResolution::Resolved(binding) => {
        reference_or_alias = Some(binding);
        is_set = true;
    }
    FallbackResolution::Unbound => {
        reference_or_alias = None;
        is_set = true;
    }
    FallbackResolution::Missing => {}
}

Instead of using Option<Option< to convey the same, but I'm not a Rust expert so I leave it up to the reviewer to decide.

[Freakazo]

I went ahead with enum DependencyResolution to make the code easier to reason about (Atlast for me)

[Freakazo]

@Boshen hoping to get a review for this