Previously, if a specifier was imported more than once, with conflicting
assertions, we'd only really consider the final import. This is wrong:
conflicting assertions are an error in-themselves already. I think this
behaviour is still wrong, because if a module is imported with incorrect
assertions later in the graph after the module has already been loaded by
an import with a correct assertion, no error will be raised.

This is a pre-requisite for WASM imports that additionally allows us to
minimize clones when doing things like BOM stripping.