Refactor the build process. Piggyback on remake's dependency tracking

to perform a topological sort of the object files, used during linking.
This is done by generating a .dep file for each object file keeping in
a .dep the transitive closure of its dependencies.
Also, packed modules must now be described by:
- a directory
- a file with the .pack extension, with the same basename as the directory,
listing the module names (relative to the directory containing the .ml files)