Reconcile type name of library instances with other custom types in the ytt standard lib

[pivotaljohn]

Describe the problem/challenge you have
Rename type(library.get()) from "module" to "@ytt:library.instance".

For example, with:

_ytt_lib/lib/template.yml
config.yml

#! _ytt_lib/lib/template.yml
---

#! ./config.lib
#@ load("@ytt:library", "library")

type: #@ type(library.get("lib"))

this happens:

ytt -f .
type: module

Describe the solution you'd like

Would rather this output:

ytt -f .
type: @ytt:library.instance

To reconcile the emerging convention of type names in the ytt standard library.

Anything else you would like to add:
Documentation refers to this string already. ;)

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