Fix bug: diff being detected for source_repo_repository even when there are no changes

[modular-magician]

This patch fixes a bug in source_repo_repository where a diff was always being generated for the TypeSet field pubsub_configs since its Set hashing function was not accounting for the fact that pubsub_configs[].topic can contain either a topic's name or relative path.

To reproduce, apply the following Terraform configuration, then run terraform plan right after. Notice how terraform plan detects a diff even when nothing was actually changed.

provider "google" {
  project = "my-project-test"
}

resource "google_pubsub_topic" "topic" {
  name = "my-topic-test"
}

resource "google_sourcerepo_repository" "repo" {
  name             = "my-repo-test"
  pubsub_configs {
    topic           = google_pubsub_topic.topic.name
    message_format  = "JSON"
  }
}

Release Note Template for Downstream PRs (will be copied)

sourcerepo: fixed perma-diff in `google_sourcerepo_repository`

Derived from GoogleCloudPlatform/magic-modules#3786