Move platform code to its own directory

[umohnani8]

Move the platform code into its own directory
under libimage so it can be used by farm build in
podman without causing the binary size to increase a lot.

[umohnani8]

Needed to reduce binary bloat size from changes in containers/podman#20050.
@vrothberg @rhatdan @Luap99 PTAL

[vrothberg]

Doesn't compile.

# github.com/containers/common/libimage/platform
libimage/platform/platform.go:75:10: undefined: Image
# github.com/containers/common/libimage
libimage/runtime.go:187:17: undefined: PlatformPolicy
libimage/runtime.go:286:54: undefined: NormalizePlatform
libimage/runtime.go:430:44: image.matchesPlatform undefined (type *Image has no field or method matchesPlatform)
libimage/runtime.go:438:8: undefined: PlatformPolicyDefault
libimage/runtime.go:440:8: undefined: PlatformPolicyWarn
libimage/copier.go:242:96: undefined: NormalizePlatform
libimage/pull.go:168:31: image.matchesPlatform undefined (type *Image has no field or method matchesPlatform)

func (i *Image) matchesPlatform(ctx context.Context, os, arch, variant string) (error, bool, error) must continue living in libimage

[vrothberg]

PlatformPolicy could be moved to libimage/define. I think this also requires an immediate vendor-PR against Podman since it's probably breaking Podman compilation.

[umohnani8]

Buildah vendor PR: containers/buildah#5085
Podman vendor PR: containers/podman#20338

[umohnani8]

Tests here and in buildah are green, PTAL. Once merged I will update the buildah vendor PR and then the podman vendor PR

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rhatdan, umohnani8

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[rhatdan]

LGTM

[rhatdan]

Of course we just broke your vendors into Buildah/Podman.

[rhatdan]

@vrothberg @Luap99 PTAL

[rhatdan]

@lsm5 @mheon @baude @ashley-cui PTAL

[mheon]

/lgtm