Remove inline metadata module hack from #1665

[badboy]

We should remove this and find a better way to handle this.

See #1665

[badboy]

From @mhammond in that PR:

I wonder if things have changed enough that could move uniffi_core/src/ffi_converter_*.rs into uniffi_meta, which might make this go away.

[badboy]

rust-lang/rust#114470 landed and might fix it, it should land in Rust 1.73, so still a bit out for us to rely on though.