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

**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
```

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

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

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

this happens:

```bash
ytt -f .
type: module
```

**Describe the solution you'd like**

Would rather this output:

```bash
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](https://carvel.dev/ytt/docs/latest/lang-ref-ytt-library/) refers to this string already. ;)

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Reproduce the example using _ytt_lib/lib/template.yml, config.yml, and `ytt -f .`; start by tracing the `library.get()` type-name handling. Done means the example reports `@ytt:library.instance` instead of `module`, consistently with the linked ytt standard-library documentation.

Written by the indexing model from the issue text.