fix(genaip): invalid code generation for top-level singletons

[vallahaye]

Hello there :wave:

This fixes the issue I encountered when generating top-level singleton resources.

The changes involve passing a protogen.GoIdent to g.P instead of using g.QualifiedGoIdent, which always imports the identifier's package, whether it's used or not.

The test I've added is essentially a dummy test to force the compilation of the generated code.

Closes #310