[wasm-split] Move shareImportableItems (NFC) (WebAssembly#8686)

This just moves the function up in the file. A follow-up PR will change
the order of functions and run it before
`indirectReferencesToSecondaryFunctions`, so I'd like to match the order
of the functions in the file to match that order, but if I move the
function in that PR, it is hard to see what changes in the function.

Author: aheejin