Ensure pulumi/pkg and pulumi/sdk references are identical

[t0yv0]

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Issue details

The release/compatibility guarantees on pulumi/pulumi are such that pulumi/pkg and pulumi/sdk Go module reference versions are assumed to match in the consuming code. If they do not match, there might be issues like the code would not compile. Make sure upgrade-provider enforces/respects this invariant.

Affected area/feature

[Frassle]

Note that tfbridge depends on pkg internals which sometimes have source code breaking changes. So upgrade-provider ought to make sure it doesn't update pkg&sdk beyond what tfbridge is currently testing and building against.