Improve parallelism when loading transformer plugins.

Rather than only looking at the package-level dependency graph, pub will now
examine transformer files' imports to determine dependencies between
transformers. This improves both load times and cycle detection. For example, a
package using polymer transformers can now load them in two phases rather than
four, reducing the load time by about half.

In addition, this gives transformer authors more control over how parallel their
transformer graph is. With some $include/$exclude and import tweaks, most
remaining transformer dependencies can be eliminated.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//331263002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37461 260f80e4-7a28-3924-810f-c04153c831b5