Support bindings wrapper crates that can be used in component implementation crates

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This is a continuation of #75, which exposes the use case. Quoting @peterhuene here:

For the feature request, I think it would be along the lines of:

1. Allow component metadata in `lib` packages that aren't `cdylib`; only generate bindings.
2. Do not attempt to componentize the output of such packages (i.e. there is no `.wasm` output).
3. Ensure that linking in multiple custom sections containing wit-bindgen metadata sourced from those rlibs is properly combined when componentizing the final component (i.e. the `cdylib` or `bin` package that link the rlibs).

_Originally posted by @peterhuene in https://github.com/bytecodealliance/cargo-component/issues/75#issuecomment-1532045573_

Inizia leggendo l’issue #75 e seguendo il modo in cui cargo-component gestisce i metadati per i pacchetti lib, cdylib e bin. Determina come rappresentare i pacchetti lib che contengono solo bindings, l’assenza dell’output .wasm e più custom sections wit-bindgen collegate; il lavoro è completato quando il componente finale combina correttamente i metadati delle rlibs collegate.

Scritto dal modello di indicizzazione a partire dal testo della issue.