The `adopt_info` field should be checked to reference a valid part.

### Bug Description

Right now the `adopt-info` field accepts any string, whether it's a part in the project or not.

### To Reproduce

Use testcraft to build the file below.

### part yaml

```shell
name: minimal-valid-project
parts: {}
version: "0.0"
adopt-info: my-part
platforms:
  amd64:
    build-on: [amd64]
    build-for: [amd64]
```

### Relevant log output

```shell
n/a
```