Make extract_package() work for toplevel classes.

Before this CL, classes in the unnamed package, such as

L$r8$backportedMethods$utility$Objects$2$checkIndex

(which looks like it is generated by R8 for Objects.checkIndex() calls)
were breaking the build because extract_package was incorrectly assuming
that there would always be a '/' in the identifier string.

Test: Build that previously broke is now working.

Change-Id: Ice78d6b31c4f38a3c9d529bc6156d625d19bcacf