Unresolved dependency warning includes source position. Fixes #528

Unresolved dependency warning is moved to UnresolvedDependencyWarning class including
the fail path that was added in #1467.
To display the source position, I need to access the State, so I had to move the
error processing out of IvyActions and add UnresolvedDependencyWarning, which is
aware of State.
Adds UnresolvedWarningConfiguration that caches ModuleID -> SourcePosition mapping.

UnresolvedWarning is moved back to IvyActions.scala where it belongs.
The mapping between ModuleID and SourcePosition is passed in as UnresolvedWarningConfiguration.
This is calculated once in Defaults using State and is cached to filesystem.
Adds `dependencyPositions` task to explicitly track dependency positions.
Merge pull request #1491 from sbt/fix/528

Unresolved dependency warning includes source position. Fixes #528