…rpc#418)

If a service solely requires types from other packages the result is
that the connect services don't have access to the filedescriptorsets.
This causes a lot of issues such as server reflection failing.

This PR blank imports the base go file that should always exist

For the proto file:

```proto
// service/eliza.proto
syntax = "proto3";
package service;
import "models/eliza.proto";

service ElizaService {
}

// models/eliza.proto
syntax = "proto3";
package ext;

message SayRequest {
  string sentence = 1;
}
message SayResponse {
  string sentence = 1;
}
```

The following diff occurs in the generated `eliza.connect.go` code
```diff
package serviceconnect

import (
	context "context"
	errors "errors"
	models "github.com/bufbuild/connect-demo/internal/gen/buf/connect/demo/eliza/v1/models"
+	_ "github.com/bufbuild/connect-demo/internal/gen/buf/connect/demo/eliza/v1/service"
	connect_go "github.com/bufbuild/connect-go"
	http "net/http"
	strings "strings"
)

```

There is no change if the base types are already in the
`service/eliza.proto` (the way we recommend using proto files)

Closes TCN-786

Prepare for the next release, and fix connectrpc#424.