npm / arborist / #912 / 1

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support resolvable partial intersecting peerSets

In cases where a peerSet _partially_ overlaps another, we were
previously treating that as a conflict, because not everything in the
peerSet was being replaced.

So for example,

```
root -> (a)
a -> PEER(w@1, x@1, y@1)
b -> PEER(x@1.0, y@1.0, z@1.0)
```

First we install, and get `w@1.1.0`, `x@1.1.0` and `z@1.1.0`.

Then, we try to install `b`, and need to install the `1.0.0` versions of
`x` and `y`.  We could replace `x` and `y`, but then `z` had no
replacement, and was treated as an ERESOLVE mistakenly.

Some work was required to make it still note when a not-replaced member
of the peer set _is_ indicative of a conflict, because it depends on a
version of a dep within the peer set that is not satisfied by the one
being replaced.

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