This is something of a quick hack to work around the limitations that
are at the root of #2310. In short, `StackReference` resources do not
change during a update because their inputs have not changed and we do
not refresh resources as part of the update by default. We want
`StackReference` to remain a resource for myriad reasons (not the least
of which is to avoid a breaking change), but it does seem correct for it
to refresh its state during each update. Because there is no actual CRUD
operation associated with a `StackReferece`, we can obtain this behavior
by changing the implementation of its ctor in the various SDKs s.t. its
options bag always contains an appropriate `id`, thus indicating a
`Read`.
Fixes#2310.
These changes add a new resource to the Pulumi SDK,
`pulumi.StackReference`, that represents a reference to another stack.
This resource has an output property, `outputs`, that contains the
complete set of outputs for the referenced stack. The Pulumi account
performing the deployment that creates a `StackReference` must have
access to the referenced stack or the call will fail.
This resource is implemented by a builtin provider managed by the engine.
This provider will be used for any custom resources and invokes inside
the `pulumi:pulumi` module. Currently this provider supports only the
`pulumi:pulumi:StackReference` resource.
Fixes#109.
