path parameters that refer to request message subfields are not mapped

protoc-gen-openapi does not map path parameters that refer to request message subfields (as in `"/categories/{book_key.category}/books/{book_key.name}"`) as expected.

As an example, running protoc like this:

```
protoc library.proto \
--proto_path=. \
--proto_path=/path/to/googleapis \
--openapi_out=. \
--openapi_opt=enum_type=string \
--openapi_opt=default_response=false
```

with this proto:

```proto
syntax = "proto3";

package library;

import "google/api/annotations.proto";

option go_package = "example.com/library";

service Library {
rpc GetBook(GetBookRequest) returns (Book) {
option (google.api.http) = {
get: "/categories/{book_key.category}/books/{book_key.name}"
};
}
}

enum Category {
CATEGORY_UNSPECIFIED = 0;
FICTION = 1;
NONFICTION = 2;
}

message BookKey {
Category category = 1;
string name = 2;
}

message GetBookRequest {
BookKey book_key = 1;
}

message Book {
}
```

produces this output:

```yaml
# Generated with protoc-gen-openapi
# https://github.com/google/gnostic/tree/master/cmd/protoc-gen-openapi

openapi: 3.0.3
info:
title: Library API
version: 0.0.1
paths:
/categories/{book_key.category}/books/{book_key.name}:
get:
tags:
- Library
operationId: Library_GetBook
parameters:
- name: book_key.category
in: path
required: true
schema:
type: string
- name: book_key.name
in: path
required: true
schema:
type: string
- name: bookKey.category
in: query
schema:
enum:
- CATEGORY_UNSPECIFIED
- FICTION
- NONFICTION
type: string
format: enum
- name: bookKey.name
in: query
schema:
type: string
responses:
"200":
description: OK
content:
application/json:
schema:
$ref: '#/components/schemas/Book'
components:
schemas:
Book:
type: object
properties: {}
tags:
- name: Library
```

Note that `book_key.category` and `book_key.name` have not been mapped to their corresponding request message subfields, which therefore also show up as query parameters (`bookKey.category` and `bookKey.name`).

As far as I know, having path parameters refer to subfields is valid. It's mentioned in [http.proto](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto) and is supported by the [Envoy gRPC-JSON transcoder](https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/grpc_json_transcoder_filter).