Private functions in imported DaphneDSL scripts

When a DaphneDSL script *A* imports another DaphneDSL script *B*, all UDFs defined in *B* become visible in *A* (with a prefix `B.`). However, some of the functions in *B* may be just internal helper functions that are not meant to be used outside. At some point in time, we should think of a way to disable or discourage the use of such internal functions.

Possible alternatives include:
- disabling the use
- some way to indicate that the UDF in *B* should not be visible in an importing script *A*; could be something like a `private` qualifier or some naming convention (e.g., starting with underscore)
- discouraging the use
- could be some naming convention (e.g., starting with underscore); whenever a user accesses `B._xyz` they know that this is discouraged, but it's still possible

**This isse is *not* urgent at the moment.**

No files, tests, or implementation entry points are named. Start by tracing how imported DaphneDSL script B exposes its UDFs to script A, then decide whether privacy should disable access or only discourage it; done requires an agreed syntax or convention and tests covering imported helper functions.

Written by the indexing model from the issue text.