Spell platforms through one module

[MayCXC]

Summary

The tree imports both github.com/containerd/containerd/platforms and github.com/containerd/platforms. The first is a deprecation shim whose every symbol forwards to the second, so the two spellings name the same code and the split lives only in the source: pkg/build reaches for one in build.go and for the other in buildopts.go and frontend.go, and a reader has to open the vendored shim to learn that they agree.

One module spells every platform now, the one dockerui itself imports and the one the shim forwards to.

Motivation and Context

Housekeeping, and I want to be precise about what it is not: I checked the vendored shim before writing this, and Format, Normalize, and DefaultSpec are one-line forwards, so no behaviour changes here and no normalization difference is being fixed. What changes is that the deprecated import path leaves the tree and a reader no longer has to establish that the two spellings agree.

Testing

• go build ./... and go test ./... pass.
• No functional diff: the four edited lines are import paths, and every symbol used through the old path is a forward to the one now imported directly.