Attempts to swp the current {#possibility} with the already-activated spec with the given name @return [Boolean] Whether the possibility was swapped into {#activated}
@return [String] the name of the source of explicit dependencies, i.e.
those passed to {Resolver#resolve} directly.
Ensures any raised {NoSuchDependencyError} has its {NoSuchDependencyError#required_by} set. @yield