ctr import: strictly match platform
ctr import: strictly match platform

Currently, ctr import will use loose matching as defined by
platforms.Only(), meaning in the case of platform linux/amd64 as in
issue#6441, importing will also match linux/386 platform on the
image-to-be-imported's index. However, that image-to-be-imported may not
have both the linux/amd64 and linux/386 platform contents, resulting in
a failure to unpack the image. This change makes that check strict such
that the requested platform to import for is the only platform content
imported. Both ctr pull and ctr export will treat the platform option as
strict, so this change makes ctr import consistent with those.

resolves #6441

Signed-off-by: Gavin Inglis <19991cc1ad5a3e4805f0cfc9963fb6df455e4f37@amazon.com>

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