Dart Plugin: have PubListPackageDirsAction create a new AnalysisContext with optimizing options to crawl/analyze the entire project to compute the set of files that should be added to the IntelliJ index.

This change unblocks users who require pub list for analysis.  There will be some cleanup after this lands, including some refactoring (Alex, is there a better way to implement getAllDartFilesFromRoot()?), I also want to modify the UI.