Use Comparators for PackSource

Use Comparators for PackSource

Rather than requiring callers to do their own computations based on the
package-private "category" number, provide an actual
Comparator<PackSource> instance, and explicitly discourage usage of
default Enum comparison.

Construct the default comparator using a builder pattern based on
defining equivalence classes. This gives us the same behavior as the old
category field in PackSource, with an abstraction that does not leak the
implementation detail of comparing rank numbers.

Change-Id: I6757211397ab1bc181d61298e073f88b69dbefc3