Issue title: Support submodules in Remote Assets
 Issue body: **Is your feature request related to a problem? Please describe.**
A git remote asset that contains a submodule should also fetch the submodule contents. Without the submodule contents the project fails to build.

**Describe the solution you'd like**
Something along the lines of `git clone --recurse-submodules` or `git submodule init; git submodule update` would fetch and populate the submodule contents

**Describe alternatives you've considered**
I've tried defining another remote asset for the submodule, with the intent of copying the contents into the proper location in %prep. E.g.

https://gitlab.suse.de/jfehlig/libvirt-rpm/-/tree/v8.0.0-sle15sp4

The corresponding IBS project is here

https://build.suse.de/package/show/home:jfehlig:gitlab-ci/libvirt

The build service seems slow to respond to my `osc service rr home:jfehlig:gitlab-ci/libvirt`. I triggered the rr to pick up the updated spec file containing the submodule remote asset but so far haven't seen any change.
 Comments: 
 Comment 0: tracked in obs-build now
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