fix: report the real file and index in kyaml input errors

[PragalvaXFREZ]

inpututil.WrapErrorWithFile is supposed to decorate a KRM function error
with the file and document index the resource came from. It currently
produces []: <error> for every input.

Two things go wrong in the same statement:

path := meta.Annotations[kioutil.PathAnnotation]
index := meta.Annotations[kioutil.IndexAnnotation]
if path == "" {
	path = meta.Annotations[kioutil.LegacyPathAnnotation]
}
if index == "" {
	index = meta.Annotations[kioutil.LegacyPathAnnotation]
}
return errors.WrapPrefixf(err, "%s [%s]",
	meta.Annotations[path],
	meta.Annotations[index])

1. path and index already hold the annotation values (deployment.yaml,
   3), but they are then used as keys in a second meta.Annotations lookup.
   That lookup misses, so both format arguments are empty strings.
2. The legacy fallback for index reads LegacyPathAnnotation instead of
   LegacyIndexAnnotation, so a resource carrying only the legacy annotations
   resolves its index to the file path.

The fix uses the values that were already resolved and reads the index from
LegacyIndexAnnotation. This is the same resolution kioutil.GetFileAnnotations
already performs correctly for the identical pair of annotations.

Anything built on inpututil.MapInputs / MapInputsE is affected: the error a
function author surfaces cannot name the offending file or document, which is
the whole purpose of the wrapper.

Before

--- FAIL: TestMapInputsEWrapsErrorWithFileAndIndex/internal_annotations
    Error: Error message not equal:
           expected: "deployment.yaml [3]: boom"
           actual  : " []: boom"

After

--- PASS: TestMapInputsEWrapsErrorWithFileAndIndex/internal_annotations
--- PASS: TestMapInputsEWrapsErrorWithFileAndIndex/legacy_annotations
ok  	sigs.k8s.io/kustomize/kyaml/inpututil	0.041s

The kyaml/inpututil package had no tests; this adds one covering both the
internal and legacy annotation forms. All 34 packages in the kyaml module
still pass.

[kubernetes-prow]

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[kubernetes-prow]

Hi @PragalvaXFREZ. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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