When outputting the names section, we should output the actual symbol
name rather than the import name. This makes sure that symbols with
an explicit name set have the correct name but retain the import name
too.
We also now correctly mangle the name of an extern function with an
explicit library name. This ensures that functions that have a
different library name, but the same import/function name, can be
resolved correctly with other modules and don't resolve to the
same symbol.
