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

[bnjbvr]

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