Typo in comment.
Partial implementation of dependency loading.

Add LoadTask.finish().

Greatly simplify the interface of @importFor, make it independent of
LinkageUnits, and rename it to @import.  Make it throw certain errors
synchronously.  Move import()-specific behavior into Loader.import().

Rough-cut implementation of error handling (LinkSet.fail(),
LoadTask.fail()).

Add .fullName property to LoadTasks.  Rename
LoadTask.linkSetsWaitingForCompile to .linkSets.