go/analysis/passes/shadow: separate package-level variables logic

[bkreitch]

Span-based shadow detection across multiple files may produce inconsistent
results due to changes in filename ordering and map iteration. Since only
package-level variables can be shadowed cross-file, this PR handles them
separately using usage-based detection.

Fixes golang/go#40758
Continues from #583

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Patch Set 1:

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Patch Set 1:

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Patch Set 2:

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Patch Set 2:

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Patch Set 3:

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Patch Set 3:

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Patch Set 3:

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