spec flatten, normalize, validate; package validate

New operations for manipulating spec and package DAGs.

For specs:
	flatten:   gather all deps to the root
	normalize: Merge constraints and make spec match package DAG

For packages:
	validate_dependencies: Make sure spec constraints in package DAG are sane.

Added tests for above methods.  Also added beginnings of concretization logic,
to turn abstract spec into a concrete one.

Still need proper tests for normalize().