artifact: only allow single manifest

[Luap99]

Allowing for multiple manifest per artifact just makes the code and cli design harder to work with it. It is not clear how mounting, extracting or edit on a multi manifest artifact should have worked.

A single manifest should make the code much easier to work with.

Does this PR introduce a user-facing change?

None

[Luap99]

cc @baude @Honny1 @mtrmac

[Honny1]

LGTM, I just have a minor, non-blocking question.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Honny1, Luap99

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[mheon]

Code LGTM but I don't want to merge without comments from @baude and/or @mtrmac

[mtrmac]

A quick take if it helps: I have no concern with this PR from c/image perspective.

I am not currently familiar with the Podman artifact code enough to have much of an opinion on the API design or other aspects of these subpackages.

[Luap99]

@baude @mtrmac PTAL again

[baude]

@Honny1 i think i am stating the obvious but this will likely impact #25179

/lgtm

[baude]

/cherry-pick v5.4

[openshift-cherrypick-robot]

@baude: #25200 failed to apply on top of branch "v5.4":

Applying: artifact: only allow single manifest
Using index info to reconstruct a base tree...
M	pkg/libartifact/store/store.go
M	test/e2e/artifact_test.go
Falling back to patching base and 3-way merge...
Auto-merging test/e2e/artifact_test.go
CONFLICT (content): Merge conflict in test/e2e/artifact_test.go
Auto-merging pkg/libartifact/store/store.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"
Patch failed at 0001 artifact: only allow single manifest

In response to this:

/cherry-pick v5.4

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