@return [Integer] The index to which the resolution should unwind in the
case of conflict.
@return [Object] the requirement that led to a version of a possibility
with the given name being activated.
Requires the dependencies that the recently activated spec has @param [Object] activated_spec the specification that has just been
activated
@return [void]
@return [String] the name of the source of explicit dependencies, i.e.
those passed to {Resolver#resolve} directly.
@return [String] the name of the source of ‘locked’ dependencies, i.e.
those passed to {Resolver#resolve} directly as the `base`