`:capabilities` endpoint returns 404 for valid Go packages

[absol27]

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

Example

# 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.