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