npm / arborist / #736

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fix: deps of linked deps in package-lock

When having optional dependencies of a dependency of
a top-level link dependency, e.g:

    root -> LINK(a)
    a -> (b)
    b -> OPTIONAL(c)

    * c marked as extraneous

Any optional dependency (and all its nested nodes) were marked
as extraneous in package-lock due to `calcDepFlags` missing
properly following the target of Link nodes.

This changeset fixes it by properly following the result of the
`treeverse.visit` method that will have already followed any
link targets at that point.

Fixes: https://github.com/npm/cli/issues/2505

PR-URL: https://github.com/npm/arborist/pull/229
Credit: @ruyadorno
Close: #229
Reviewed-by: @isaacs

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