feat: in standalone mode (without etcd), protobuf with import is not loaded

### Current Behavior

When provide config and protobuf files using [standalone mode](https://apisix.apache.org/docs/apisix/deployment-modes/#how-to-configure-protos), and configured with a proper protobuf with `import`, the protobuf will not be loaded with error `No such file or directory`.

### Expected Behavior

_No response_

### Error Logs

_No response_

### Steps to Reproduce

Try use this proto, and reference it in a route:

```protobuf
protos:
- id: helloworld2
desc: hello world
content: >
syntax = "proto3";
package helloworld2;
import "helloworld.proto";

service Greeter {
rpc SayHello (HelloRequest) returns (HelloReply) {}
}
message HelloRequest {
string name = 1;
}
message HelloReply {
string message = 1;
}
- id: helloworld
desc: hello world
content: >
syntax = "proto3";
package helloworld;

service Greeter {
rpc SayHello (HelloRequest) returns (HelloReply) {}
}
message HelloRequest {
string name = 1;
}
message HelloReply {
string message = 1;
}
#END
```

### Environment

- APISIX version (run `apisix version`): 2.x

Reproduce the failure using the standalone-mode configuration and protobuf example in the issue, then trace the protobuf loading path for imported files. Done means a protobuf that imports helloworld.proto loads successfully and can be referenced by a route without a “No such file or directory” error.

Written by the indexing model from the issue text.