aip0124: why do referenced resources need to be in the same package?

The implemented rule for AIP124 [enforces](https://github.com/googleapis/api-linter/blob/master/rules/aip0124/valid_reference.go#L91) that if a resource is referenced using `google.api.resource_reference` annotation, the referenced resource must be in the same package as the one referencing it. Otherwise, the linter will lint it as **"resource not found"**.

Why is this implemented this way ?? This requirement is not mentioned in [AIP124](https://google.aip.dev/124).

I think an explanation for this should be given in AIP124 or the requirement to be in the same package removed from the implementation of the rule.

Just to make it a bit clearer, here's an example. If we have 2 proto files, `dep.proto`, and `leaf.proto` (taken from the testing inside rule 124) like:

"dep.proto"
```
import "google/api/resource.proto";
package AAA
message Book {option (google.api.resource) = {
type: "library.googleapis.com/Book"
}
```
"leaf.proto": `
```
import "google/api/resource.proto";
import "dep.proto";
package BBB
message Foo {
string book = 1 [(google.api.resource_reference).{{.Field}} = "{{.Ref}}"];
}
```

AAA must be equal to BBB otherwise, the resource will be lint as not found.