ImportData imports a package by reading the gc-generated export data, adds the corresponding package object to the imports map indexed by id, and returns the object.
The imports map must contains all packages already imported. The data reader position must be the beginning of the export data section. The filename is only used in error messages.
If imports[id] contains the completely imported package, that package can be used directly, and there is no need to call this function (but there is also no harm but for extra time used).