fix: move pathological nest fixing link to PlaceDep

[isaacs]

When a dependency graph cycles back on itself incompatibly like this:

a@1 -> b@1
b@1 -> a@2
a@2 -> b@2
b@2 -> a@1

We would find ourselves unable to handle the conflict via nesting. For
example:

root
+-- a@1 -> b@1
+-- b@1 -> a@2
    +-- a@2 -> b@2
    +-- b@2 -> a@1
        +-- a@1 -> b@1
        +-- b@1 -> a@2
            +-- a@2 -> b@2
            +-- b@2 -> a@1 (cycling forever)

In order to address this, we create a link when such a cycle is
detected.

root
+-- a@1 -> b@1
+-- b@1 -> a@2
    +-- a@2 -> b@2
    +-- b@2 -> a@1
        +-- a@1 -> b@1
        +-- b@1 -> link to root/node_modules/b@1

Prior to the recent refactor to move much of the dependency placement
logic out of Arborist.buildIdealTree and into the PlaceDep class, this
link was created right at the moment when a new dependency was created
in a temp tree.

However, if we feed that Link object into the PlaceDep flow, it will
(correctly) see that the Link does not match the Node further up the
tree, and attempt to replace it.

Compounding the problem (and why it appeared in npm dedupe and not
npm install) is the fact that explicitly named updates are always
treated as a "problem edge", so that they can be re-evaluated.

So, rather than creating a Node to be tested against the tree, it was
creating a Link object, and then attempting to replace the Link's target
with the Link itself, which caused some havoc.

This patch moves the loop detection and remediating Link creation into
the PlaceDep class, which is the more proper place for it, as that class
owns the "put deps into the tree" logic, and this is clearly a "put deps
into the tree" type of situation.

Via: @ParadoxInfinite
Close: npm/cli#3632
Close: #308
Fix: npm/cli#3565

References

[nlf]

this looks good! makes sense to me