npm / arborist / #363

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Do not conflict on meta-peers that will not be replaced

When in a situation where a project has dependencies on several members
of a peer set, it was encountering a spurious ERESOLVE error if the
entire set was being replaced, but _one_ of the members did not need to
be.

For example:

```
root -> (a@1, b@1, c@1)
a@1 -> PEER(b@1, c@1)
a@2 -> PEER(b@2, c@1)
b@1 -> PEER(c@1)
b@2 -> PEER(c@1)
```

resulting in the tree:

```
root
+-- a@1
+-- b@1
+-- c@1
```

If we try to upgrade both `a` and `b` to version 2, however, we would
check the set of peers for each dependency node being replaced, and find
that there was a `c` node with a non-peer dependency from the root node,
and treat it as a conflict, even though there was no need for `c` to be
modified at all!

Resolve this by skipping the check when not doing `canPlacePeers` (since
it's either already a conflict, or already set to be overridden by the
version from the virtualRoot), and ignoring the conflict when a
non-overridden node exists in the virtualRoot which meets the non-peer
edge keeping the current dependency node in place.  In those cases,
either we will replace or keep the current node anyway, so there's no
need to conflict on it.

Fix: npm/cli#2000

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