cmd/go/internal/modfetch: prevent duplicate hashes in go.sum

[powersj]

To write go.sum, each module and then each hash is looped through. The
hashes are kept in a slice and there is no check to ensure that hashes
were not added or already exist in the file. Therefore, unique the
hashes of each module before writing to prevent duplicates.

Fixes: #28456

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This PR is being closed because golang.org/cl/411154 has been merged.