npm / arborist / #724 / 1

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Allow --force to override conflicted peerOptional

With a dependency graph like this:

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

We do not install the peerOptional dependency by default, so even though
`b@2` is included in the peerSet of `a`, it is not added to the tree.

Then, the `b@1` dependency is added to satisfy root's direct dependency
on it, causing the `a -> b@2` edge to become invalid.

We then try to resolve the `a -> b@2` edge, and find that we cannot
place it anywhere, causing an `ERESOLVE` error.

However, because `b@2` is no longer a part of a peerSet sourced on the
`root` node, we miss the chance to detect that it should be overridden,
resulting in an `ERESOLVE` failure even when `--force` is used.

This commit adds the check for `this[_force]` prior to crashing with
ERESOLVE, so that cases that avoid our earlier heuristics still accept
the invalid resolution when `--force` is in effect.

Fix: #226
Fix: https://github.com/npm/cli/issues/2504

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