libpod/image: Use ParseNormalizedNamed in RepoDigests

Avoid generating
quay.io/openshift-release-dev/ocp-release@sha256@sha256:239... and
similar when the image name is already digest-based [1].  It's not
clear exactly how we get into this state, but as shown by the unit
tests, the new code handles this case correctly (while the previous
code does not).

[1]: https://github.com/containers/libpod/issues/2086

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #2106
Approved by: rhatdan
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