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Do not ERESOLVE on peerOptionals not added to tree

For these dependency graphs:

    # case a
    root -> (x, y@1)
    x -> PEEROPTIONAL(z)
    z -> PEER(y@2)

    # case b
    root -> (x) PEEROPTIONAL(y@1)
    x -> PEEROPTIONAL(y@2)

    # case c
    root -> (x) PEEROPTIONAL(y@1)
    x -> PEER(z)
    z -> PEEROPTIONAL(y@2)

The peerOptional dependency is included in the peerSet, which would
raise an ERESOLVE conflict at the peerSet generation stage, even though
the peerOptional dependencies will not actually be added to the tree.

To address this, this commit tracks the nodes which are actually
required in the peerSet generation phase, by virtue of being
non-optionally depended upon by a required node in the peerSet.

If a conflict occurs on a node which is not in the required set during
the peerSet generation phase, we ignore it in much the same way that we
would ignore peerSet errors in metadependencies or when --force is used.

Of course, if the peerOptional dependency is _actually_ required, to
avoid a conflict with an invalid resolution present in the tree already,
and there is no suitable placement for it, then ERESOLVE will still be
raised.

Fix: #223
Fix: https://github.com/npm/cli/issues/2667

PR-URL: https://github.com/npm/arborist/pull/227
Credit: @isaacs
Close: #227
Reviewed-by: @ruyadorno

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