Make it possible to place models in a different package to prevent circular package references for go-server

[Kraust]

Continuing the work started in #9089. After merging master into the original branch I discovered several other pieces that needed updating that lead me to create #11622 and refactor a bit of the code (the supporting files are now in their own directory as they're shared by both the APIs and Models).

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[wing328]

please use 4-space instead of tabs

[jdemchuk]

I’m interested in this PR, what needs to be resolved in order to complete checks and merge?