Improve display/handling of pub packages in solution tree

[DanTup]

Pub packages work a bit like NuGet packages in Dart; and live in the packages folder.

The list of packages is stored in pubspec.yaml in the dependencies section. This only lists the packages you directly add; dependencies of dependencies will live in the packages folder even though they're not in this file.

Currently, the folders inside the packages folder (one per package) are symlinks back to a "pub cache" (eg. C:\Users\Danny\AppData\Roaming\Pub\Cache) though this may change in the future.

Additionally, pub creates packages symlinks as subfolders of many other folders (eg. bin, web). This means rendering the entire tree will have a bunch of duplicated folders:

• bin
• - packages
• web
• - packages
• packages
• pubspec.yaml

I think it might be best for us to:

1. Render the packages folder as the references folder in the current implementation (so it appears at top, and is "special", allowing for future functionality like a pub package search)
2. Hide all further nested packages folders from the solution tree
3. Indicate the difference between packages in pubspec.yaml vs those that just exist on disk (indirect dependencies)