chore: generate explicit type exports for structural interfaces

[kuhe]

This PR splits the contents of the generated client models directory.

Formerly/currently, the models directory has:

• an index, re-exporting everything
• models_0 to models_N files containing a mix of enums, interfaces, and error classes
• a base exception (synthetic)

The PR changes this to:

• no more index, instead the symbols are exported by the package index directly
• models_N now only contain types (interfaces)
• enums and errors have been moved to separate files

This allows explicit type exports, as can be seen in the package index diff in the PR.