These changes refactor direct interactions with the Pulumi API out of
the cloud backend and into a subpackage, `pkg/backend/cloud/client`.
This package exposes a slightly higher-level API that takes care of
calculating paths, performing HTTP calls, and occasionally wrapping
multiple physical calls into a single logical call (notably the creation
of an update and the upload of its program).
This is primarily intended as preparation for some of the changes
suggested in the feedback for #1067.
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