Implement a LibraryContainer / Package subclass that is a single directory

[jcollins-g]

With the refactor extracting generic LibraryContainer from Package and driving documentation generation through Package rather than PackageGraph, it's now possible to implement a LibraryContainer subclass that handles a single directory of dart code.

Implement this with a flag guard to override generic package handling (to deal with situations where things that look like real packages are checked into a one big repo).