A simple heuristic (i.e. replacing the `libraries/` suffix by
`packages/` in the inferred remotepath, if the remotepath in the
`packages` file is "-") used to infer the remotepath for Git
submodules. A proper solution would require to parse the `.gitmodules`
file to obtain the actual localpath<->remotepath mapping.

This addresses #8121 and is a follow-up to a22e908a which changed the
`.gitmodules` urls to use the `/packages/` folder instead of
`/libraries/`.