Move pub into sdk/lib/_internal.

This also converts pub to a more package-like layout, which will help in the
eventual exposing of pub internals as a library. Since pub's sources are now in
lib/src, this also makes them importable from the tests and entrypoint using
"package:" imports.

BUG=6619

Review URL: https://codereview.chromium.org//14297021

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@21907 260f80e4-7a28-3924-810f-c04153c831b5