cmd/go: report importer-specific positions in go list dependency errors

[fenghaojiang]

When multiple packages import the same missing dependency, go list may
reuse a shared PackageError and report the same Pos for every importer.
This causes incorrect locations in DepsErrors and confuses downstream
tools that rely on precise positions.

This change records whether a PackageError position refers to the
importer side, then in collectDepsErrors clones dependency errors per
importer. For importer-side errors, it rewrites Pos using the current
package's ImportPos for that dependency (with short path formatting),
so each importing package gets its own correct location.

Fixes #78183.

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