Catchup with ConstantPathNode change

[tompng]

In prism-0.28.0, ConstantPathNode#child is deprecated

Prism.parse('A::B').value.statements.body.first.child
# DEPRECATED: ConstantPathNode#child is deprecated and will be removed in the next major version. Use ConstantPathNode#name/ConstantPathNode#name_loc instead. Called from (irb):1:in `<top (required)>'.
=> 
@ ConstantReadNode (location: (1,3)-(1,4))
└── name: :B

And ConstantPathNode#child does not appear in child_nodes anymore.

Prism.parse('A::B').value.statements.body.first.child_nodes
=>
[@ ConstantReadNode (location: (1,0)-(1,1))
 └── name: :A]

# it was
[@ ConstantReadNode (location: (1,0)-(1,1))
 └── name: :A]
[@ ConstantReadNode (location: (1,3)-(1,4))
 └── name: :B]

So completion target node search for constant path completion is failing now. This pull request fixes it.

[st0012]

Nit: Feels like this could be a helper method?