Dart actually expects package: to work. This CL makes package:foo
map to /packages/foo, similar to how Dartium or bin/dart would expect.
This also means overlaying the /gen directory over the actual package
outputs (as consumers of an SDK would expect) as well as adding
an additional /lib indirection for the actual package source as
the Dart pub tool will expect.
This is far from perfect, but it unlocks us actually producing a
sky SDK.
I expect there may be some fallout from this change as I'm sure I
missed some package: uses. We also don't have a general solution
for all /foo/bar/baz includes which randomly included parts
of mojo's source directory. Those will need to be updated to use
a package: and deploy_sdk.py taught how to build a package for them.
R=abarth@chromium.org
Review URL: https://codereview.chromium.org/990493002
