Plugin Marketplace clone does not pull submodules

### Describe the bug

When adding a custom Marketplace, the clone of the target marketplace repo does not clone subrepositories. This means that the marketplace cannot utilize submodules as "pointers" to plugins in other repos, which prevents a "curated" marketplace capability.

### Affected version

1.0.6-1

### Steps to reproduce the behavior

1. Create a marketplace with a submodule that points to a plugin. For example, add the `awesome-copilot` repo as a submodule to the marketplace,
2. In the marketplace manifest, point to a skill within a plugin of the subrepo - for example `./curated/awesome-copilot/plugins/csharp-dotnet-development/skills/dotnet-best-practices`
3. Add the marketplace into GHCP - this will work w/o issue, and show the plugin you created to point at the awesome-copilot skill without issue
4. Add the plugin you created to GHCP - this will also work without issue **but the skill will not activate in GHCP**

### Expected behavior

Everything defined in any submodules (recursively) in a marketplace is available to GHCP

### Additional context

_No response_