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

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_