fix: disambiguate colliding WIT import names

[jsturtevant]

Stacks on #1331.

When two WIT packages export an interface with the same short name, the generated Rust trait has duplicate members and fails to compile. For example:

// a:pkg/types and b:pkg/types — both named "types"
world bindgen-test-cases {
    import a:pkg/types;
    import b:pkg/types;
}

After this fix, the full namespace is prepended per component to produce unique names:

impl BindgenTestCasesImports for MyHost {
    type APkgTypes = Self;   // was: type Types (duplicate!)
    fn a_pkg_types(&mut self) -> &mut Self { self }

    type BPkgTypes = Self;   // was: type Types (duplicate!)
    fn b_pkg_types(&mut self) -> &mut Self { self }
}

Two imports from the same package at different versions — the version is appended:

world my-component {
    import a:pkg/types@1.0.0;
    import a:pkg/types@2.0.0;
}

impl MyComponentImports for MyHost {
    type APkgTypesV100 = Self;
    fn a_pkg_types_v1_0_0(&mut self) -> &mut Self { self }

    type APkgTypesV200 = Self;
    fn a_pkg_types_v2_0_0(&mut self) -> &mut Self { self }
}

Also fixes a secondary collision where namespace components themselves contain hyphens (e.g. a:b-c/types vs a-b:c/types). Flat-joining with - collapsed both to ABCTypes; per-component first-letter capitalisation now gives ABcTypes vs AbCTypes.

[jsturtevant]

This will be ontop of #1331