support hyperlinking to package references in non-dart files

@ussuri, @keertip

Add support for hyperlinking to package references in non-dart files. So things like "packages/browser/dart.js" in html files. Also, refactoring the package_mgmt folder into fewer files. And, use the new dependencies code to locate the PubManager for the current workspace without it having to be passed along as an arg through lots of methods.