[Feature] Add support for makeshdeps symbols file

Each (C/C++) shared library binary package has to have 2 files to describe the dependencies of the shared library has on other shared libraries and the list of exported symbols file:
- shlibs
- symbols

As of now, debcraft only generates the shlibs file without the symbols file.

Start by locating debcraft's existing generation of the shlibs file and any tests covering shared-library binary packages. Trace how exported symbols are discovered and packaged, then add coverage showing that a symbols file is generated alongside shlibs with the package's exported symbols.

Written by the indexing model from the issue text.