npm / arborist / #770

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Fix additional peerOptional conflict cases

case D

```
root -> (x, z@1)
x -> PEEROPTIONAL(y)
y -> PEER(z@2)
```

case E

```
root -> (x, z@1)
x -> PEEROPTIONAL(y) PEER(z@1)
y -> PEER(z@2)
```

case F

```
root -> (x, z@1)
x -> PEEROPTIONAL(y) PEER(z@1) PEER(w@1)
w -> PEER(z@1)
y -> PEER(z@2)
```

Case F properly reproduces the failure seen by installing
ng-packagr@11.1.3 alongside typescript@4.

Fix: #236

cc: @kyliau

PR-URL: https://github.com/npm/arborist/pull/237
Credit: @isaacs
Close: #237
Reviewed-by: @isaacs

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