npm / arborist / #311

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fix: workspaces cleaning up nm folders

When installing from a tree that has actual/virtual trees arborist was
missing info on workspaces during build-ideal-tree leading to a reify
diff that missed all workspaces and causing all workspaces and their
dependencies to be removed from the node_modules folder.

This fixes it by making sure we append workspaces info first thing upon
reading root node in build-ideal-tree

Fixes: https://github.com/npm/cli/issues/1784

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