When referring to other Protobuf schemas from a schema, only references are required and import statements are ignored.

# What happened?

When creating a Protobuf schema with references, types from the referred schemas can be used even without actually importing them in the schema itself.

1. Create schema A by sending a POST request over `subjects/mySubjectA/versions` with the following body:
```json
{
"schema": "syntax = \"proto3\";\nmessage MyType {\n int32 id = 1;\n}",
"schemaType": "PROTOBUF"
}
```
2. Create schema B by sending a POST request over `subjects/mySubjectB/versions` with the following body:
```json
{
"schema": "syntax = \"proto3\";\nmessage MyRecord {\n string f1 = 1;\n MyType f2 = 2;\n}",
"schemaType": "PROTOBUF",
"references": [
{
"name": "mySubjectA",
"subject": "mySubjectA",
"version": 1
}
]
}
```

Creating schema succeeds without import statement.
Creating fails, if you remove the references.
This would imply that import statement is ignored and only the references are read.

# What did you expect to happen?

Creating schema would fail if types are used from the referred schema without actually importing.

The following test should throw an exception but doesn't:

```python3

def test_invalid_import() -> None:
plan = """\
syntax = "proto3";

package my.package;

message ExternalMessage {
string field = 1;
}
"""

customer_plan_event = """\
syntax = "proto3";

package another.package;

message CustomerPlanEvent {
message Created {
my.package.ExternalMessage message = 1;
}
}
"""

no_ref_schema = ValidatedTypedSchema.parse(SchemaType.PROTOBUF, plan)
dep = Dependency("this_is_ignored", Subject("this_also"), 1, no_ref_schema)
ref_schema = ValidatedTypedSchema.parse(SchemaType.PROTOBUF, customer_plan_event, None, {"foobar": dep})
```

We are apparently ignoring both the name and the subject and we are relying on the fact that the reference map is in sync with the import statements

# What else do we need to know?

Tested only with Protobuf schemas, did not try with other schema types.