Example demonstrating how to allow duplicate dependencies

[jspahrsummers]

Depends on #44.

npm has a model like this, where projects shouldn't necessarily be deduplicated. This could be implemented by allowing two equivalent ArbiterProjectIdentifiers to compare as unequal, so that they will get resolved separately.

For example, the user could encode information about dependents into the ArbiterProjectIdentifier so projects originating from different dependents are considered different.