`:capabilities` endpoint returns 404 for valid Go packages

While testing the `v3alpha` API as part of GUAC integration https://github.com/guacsec/guac/issues/2047 , I noticed the `:capabilities` endpoint returns 404 for valid Go packages and versions even though other endpoints like `:dependents` work fine.

### Example

```bash
# This works
curl 'https://api.deps.dev/v3alpha/systems/go/packages/github.com%2Fguacsec%2Fguac/versions/v1.0.0:dependents'

# This returns 404
curl 'https://api.deps.dev/v3alpha/systems/go/packages/github.com%2Fguacsec%2Fguac/versions/v1.0.0:capabilities'
```
Also tested with github.com/antlabs/deepcopy. The website shows Capabilities under the analysis tab for the same modules, it's just not available via the API.