Support getting rapids-cmake via git clone

[robertmaynard]

Description

This allows projects that need git information of rapids-cmake to get it while still using RAPIDS.cmake.

Fixes #554

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[robertmaynard]

/merge