fix(arborist): dependencies from registries with a peerDependency on a workspace

[ixalon]

Currently if you attempt to install a package from a registry that has a peer dependency on a package published from a workspace in your local project, you are presented with the error (even if all specs match and there are no version conflicts):

44 verbose stack TypeError: Cannot set property 'parent' of null
44 verbose stack     at Arborist.[nodeFromEdge] (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/node_modules/@npmcli/arborist/lib/arborist/build-ideal-tree.js:1129:17)
44 verbose stack     at async Arborist.[loadPeerSet] (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/node_modules/@npmcli/arborist/lib/arborist/build-ideal-tree.js:1373:23)
44 verbose stack     at async Arborist.[buildDepStep] (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/node_modules/@npmcli/arborist/lib/arborist/build-ideal-tree.js:976:11)
44 verbose stack     at async Arborist.buildIdealTree (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/node_modules/@npmcli/arborist/lib/arborist/build-ideal-tree.js:218:7)
44 verbose stack     at async Promise.all (index 1)
44 verbose stack     at async Arborist.reify (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/node_modules/@npmcli/arborist/lib/arborist/reify.js:153:5)
44 verbose stack     at async Install.exec (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/lib/commands/install.js:156:5)
44 verbose stack     at async module.exports (/Users/chris/.nvm/versions/node/v14.18.3/lib/node_modules/npm/lib/cli.js:78:5)

This is an edge case, but something we've come across frequently due to having multiple repos publishing packages (e.g. strongly typed clients) where occasionally a cyclic dependency exists within the type system (which TypeScript can handle once we have the packages installed).

This does not affect packages referred to by path as these are handled by: https://github.com/npm/cli/blob/latest/workspaces/arborist/lib/arborist/build-ideal-tree.js#L1236-L1238

The issue appears to be https://github.com/npm/cli/blob/latest/workspaces/arborist/lib/arborist/build-ideal-tree.js#L1245-L1247 where we find a suitable existing node, but then immediately throw it away, returning the unresolved edge.to.

I believe this is a typo and returning existingNode is the desired behaviour? I've added a test case that fails without this change.

[fritzy]

LGTM