This change introduces an abstraction for a `backend` which manages
the implementation of some CLI commands. As part of defining the
interface, we introduce a new local backend implementation that just
uses data local to the machine.
This will let us share argument parsing and some display information
between the local case and the pulumi.com case in the CLI. We can
continue to refine this interface over time (e.g. today we have the
implementation of the Destroy/Update/Preview actually writing output
but instead they should be returning strongly typed data that the CLI
knows how to display and is unified across Pulumi.com deploys and
local deploys).
But this is a good first step.
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