ref(backup): Standalone dependencies calculation

Prior to this change, the `sort_dependencies` function was the only
public dependency calculation that the import/export system used, merely
returning a list of models in dependency order. This change splits part
of that functionality out, exposing an intermediate state where we can
see all of the model relations for a given model, including which fields
they occupy on the main model. This will be useful when we add
"foreign-key remapping" capabilities on import.

To facilitate this, we have added a couple of fixtures that capture the
state of the model dependency graph in source control, in both detailed
and flattened form. These serve a few purposes:

1. It maintains a record in source control of model dependency graph
   changes.
2. It notifies team-ospo of any such changes, allowing us to ensure that
   they don't break import/export functionality.
3. It serves as a simple golden test for the dependency resolution
   mechanism.

Issue: getsentry/team-ospo#171

bin/generate-model-dependency-fixtures

@@ -0,0 +1,53 @@
+#!/usr/bin/env python
+from __future__ import annotations
+
+from sentry.runner import configure
+
+configure()
+
+import click
+
+from sentry.backup.dependencies import DependenciesJSONEncoder, dependencies, sorted_dependencies
+from sentry.testutils.factories import get_fixture_path  # noqa
+
+encoder = DependenciesJSONEncoder(
+    sort_keys=True,
+    ensure_ascii=True,
+    check_circular=True,
+    allow_nan=True,
+    indent=2,
+    encoding="utf-8",
+)
+
+
+@click.command()
+def main():
+    """Used for generating fixtures for the model dependency map."""
+
+    click.echo("\nThis script can take up to 30 seconds, please be patient...\n")
+
+    # Do all of the calculation before any file writes, so that we don't end up with partial
+    # overwrites.
+    detailed = dependencies()
+    flat = {k: v.flatten() for k, v in detailed.items()}
+    sorted = sorted_dependencies()
+
+    det_path = get_fixture_path("backup", "model_dependencies", "detailed.json")
+    with open(det_path, "w+") as fixture:
+        fixture.write(encoder.encode(detailed))
+
+    flat_path = get_fixture_path("backup", "model_dependencies", "flat.json")
+    with open(flat_path, "w+") as fixture:
+        fixture.write(encoder.encode(flat))
+
+    det_path = get_fixture_path("backup", "model_dependencies", "sorted.json")
+    with open(det_path, "w+") as fixture:
+        fixture.write(encoder.encode(sorted))
+
+    click.echo(
+        f"\nSuccess! The dependency mapping fixtures at {[det_path, flat_path]} were updated.\n"
+    )
+
+
+if __name__ == "__main__":
+    main()