Summary of main changes:
- Now makes use of luarocks dependency resolution (builds will fail if
rockspec dependencies are unmet)
- Renamed argument `external_deps` -> `exernalDeps` and add
functionality to handle external dependencies that are multiple-output
derivations
- Added an `extraVariables` argument for appending to the contents of
luarocks config `variables` table
- The `rockspecFilename` argument default is now actually used
- The `disabled` argument can now be overriden with a less-restrictive
check, as it now just sets `meta.broken` instead of throwing an error
during eval
- The `doCheck` argument is now actually honored if set to `true`
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