Fix @mtkmodel independent variable generation

[hersle]

This should fix https://discourse.julialang.org/t/how-do-i-import-t-nounits-with-a-different-name-than-t-in-modelingtoolkit/118640.

The issue was that the @mtkmodel macro internally generates independent variables as "variables", and not "parameters" (which @independent_variables does). I have tried to fix this in a way which is consistent with the rest of the code.

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API