* Use --out-rpc, rather than --out-provider, since rpc/ is a peer to provider/.
* Use strongly typed tokens in more places.
* Append "rpc" to the generated RPC package names to avoid conflicts.
* Change the Check function to return []mapper.FieldError, rather than
mapper.DecodeError, to make the common "no errors" case easier (and to eliminate
boilerplate resulting in needing to conditionally construct a mapper.DecodeError).
* Rename the diffs argument to just diff, matching the existing convention.
* Automatically detect changes to "replaces" properties in the PreviewUpdate
function. This eliminates tons of boilerplate in the providers and handles the
90% common case for resource recreation. It's still possible to override the
PreviewUpdate logic, of course, in case there is more sophisticated recreation
logic necessary than just whether a property changed or not.
* Add some comments on some generated types.
* Generate property constants for the names as they will appear in weakly typed
property bags. Although the new RPC interfaces are almost entirely strongly
typed, in the event that diffs must be inspected, this often devolves into using
maps and so on. It's much nicer to say `if diff.Changed(SecurityGroup_Description)`
than `if diff.Changed("description")` (and catches more errors at compile-time).
* Fix resource ID generation logic to properly fetch the Underlying() type on
named types (this would sometimes miss resources during property analysis, emitting
for example `*VPC` instead of `*resource.ID`).
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