RFE: Add ability for users to un-retract or re-import packages

[NeilHanlon]

As a user, I want to be able to manage the state of a package after it has been retracted--i.e., to either un-retract it, or to simply re-import that same package into another discrete import operation.

[mstg]

Just a heads up: TSC members should be able to use retrigger.py for now