API calls agains the Pulumi service may start setting a new header,
`X-Pulumi-Warning`. The value of this header should be presented to
the user as a warning.
The Service will use this to provide additional information to the
user without having the CLI have to know about every specific warning
path.
* Have backend.ListStacks return a new StackSummary interface
* Update filestake backend to use new type
* Update httpstate backend to use new type
* Update commands to use new type
* lint
* Address PR feedback
* Lint
This renames the backend packages to more closely align with the
new direction for them. Namely, pkg/backend/cloud becomes
pkg/backend/httpstate and pkg/backend/local becomes
pkg/backend/filestate. This also helps to clarify that these are meant
to be around state management and so the upcoming refactoring required
to split out (e.g.) the display logic (amongst other things) will make
more sense, and we'll need better package names for those too.
